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Old 05-21-2022, 09:55 AM   #1
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Question I'm ashamed to admit I know nothing about flushing my tanks

Okay, so this is mostly my fault. The dealership that we bought our fifth wheel Montana from said that they would do a walk-through during delivery and expect it to take about two hours. That walk-through lasted all of 15 minutes. Obviously, having never owned an RV and that was not a sufficient walk-through. I did complain and somebody came out and did another week walk-through. This is one of those situations where you don’t know what you don’t know. I had no idea what questions to ask. I’ve taken it out once and it wasn’t a pleasant experience because we had one of the worst storms in a while that lasted for three days, so I basically just sat in the RV and watched TV.
Since then I have educated myself about a lot of things but the one thing that I haven’t done yet (please stop laughing) is flushed the gray and black tanks. Where I took it to did not have sewage at the sites. No I’m getting ready to take it out again and I know I need to flush those tanks, and where I’m going to flush them is not an issue.
I am reading through the 2019 manual (which infuriatingly gives very little information and just tells you to go to the vendor site of the device you are wondering about)
There is a section about the no fuss flush option and where it supposed to be and how it supposed to be labeled. I have a hose hook up that says “tank flush“ with a caution above it it says “do not use unless the full Way termination valve is in the open position“.
I have no idea what this connection and functionality is for.

I’m also looking for information on this a switch that says “tank fill power flush“ and has a switch that says either “normal flow“ or “Fill”.
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Old 05-21-2022, 10:42 AM   #2
Carl n Susan
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The "No Fuss" or "tank flush" system is for the black tank. It is a separate hose bib connection in the convenience center which sends water through a sprayer into the black tank. This helps to wash out "stuff" remaining after it has been dumped. The warning sticker wants you to be sure the dump valve for the black tank is OPEN when you run the tank flush. Failure to open the valve results in filling the tank and either it bursts ($$$$$) or there is an eruption of sewer content through the roof vent (hilarious for the neighbors). Experienced users do run the flush with the valve closed for a short time but it isn't without risk.

The "normal fill" or "tank fill" switch is associated with the shore water input system (not a flush system). This is how you get water into the coach. Normally you hook the water hose from the park supply to this hose bib and this provides water to the coach faucets/outlets when in the "normal fill" position. The "tank fill" position sends the input water to the fresh water holding tank. This is for when you want to use the onboard water pump to supply water to the coach.

Doing a Search on any of these terms here will provide you with hours of entertainment and education of what to do and what not to do.
Carl (n Susan)
There is more to life than fuel mileage.
2012 Montana 3700RL Big Sky Package towed by a 2015 Ford F350 6.7L PSD 4WD CC LWB

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Old 05-21-2022, 11:12 AM   #3
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With the neighbors that I have, and eruption of sewage from the roof of my RV would provide them with months of entertainment. I am going to assume then that this tank flush input is either the no fuss flush or something close to it that I would use after emptying the tanks. Specifically the black tank. I do searches on this and read the manual. That’s my default and this board is my fallback when I wanna make absolutely certain that I am not making mistakes.
Thank you
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Old 05-21-2022, 11:45 AM   #4
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Kid, I apologize if this is too simplistic, but I’m going from the assumption you are very new to this and don’t have experienced friends to help. First, in order for your sewer chemicals to work, the enzymes need some time and liquid. When our tank is empty, we add about 3-5 gallons of water and then dump in our chemicals. That way, solids are in liquid to start breaking down and you don’t get the pyramid under the toilet. We have SeaLevel gages and usually dump when we are about 80% full. This allows the enzymes to break down toilet paper and waste. When dumping, we connect our sewer hose and dump our black tank first. Our hose has a clear section so we can see when the tank is clean. This is when you would want to connect a garden hose to the black tank flush outlet. Let it run until it’s clear water coming out. We close that tank. Next, we dump our two gray tanks. This cleans out the inside of the sewer hose. Close all tanks. Then, we usually prep our toilet so it is ready. This is how we have done it for the past 18 years with our different Montana’s. The one caveat is that we did a multi day PDI and knew all of our systems had been checked. If you recall way back when you were first looking at buying we said the dealer was critical in your enjoyment. Our walk through on our 2021 was several days long due to a satellite issue, but our dealer had filled all tanks, dumped them and went through every item on our PDI list (which we gave them before delivery). I am not sure if you have signed up for any rallies, but that is a good way to gain many year’s worth of experience without the pain. All of us started somewhere. The lessons are either learned by someone else’s misfortune or our own. Best of luck.
Ron and Terrie Ames with Sirius - MOC #1920
2021Montana 3230CK Super Solar Legacy Package
2021 Ram 3500 Laramie Longhorn, BIM Charging
4x4, SRW, LB, Crew Cab, Pullrite 3900 Hitch
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Old 05-21-2022, 12:27 PM   #5
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Something Ron and Carl I don’t think mentioned. Your black tank you need lots of water and little paper. I like to fill the black tank almost full before I dump. More water to flush the tank.
Lynwood Harrell
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Old 05-21-2022, 12:33 PM   #6
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What the guys above have said.
I would only add, when you use your black tank flush, keep an eye on the black plastic anti siphon vacuum breaker valve under the bathroom sink. It will be attached to two white PEX pipes.
They are notorious for failing/leaking. When you turn the water on to use the flush system, have DW watch for leaks under the sink.
Also mentioned above, get clear elbows for your sewer hose so you know when it's done.
Daryl and Marianne,
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Old 05-21-2022, 12:37 PM   #7
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For the black tank flush the procedure is pretty straight forward, but everyone develops their own particular method. I suggest you get a short clear extension to attach to your sewer outlet. Then connect your sewer hose to that. That way you can see what is coming out. Also get a dedicated water hose for the black tank flush.
Here is my procedure as a starting place.
- connect water hose to tank flush connection
- open black tank valve and drain black tank
- turn on water to the flush connection
- close the black tank drain valve for 2-3 minutes
- open the black tank drain valve (you will see more “stuff” come out thru the clear extension)
- close the black tank drain valve for 2-3 minutes
- open the black tank drain valve (you will see more “stuff”
- repeat sequence another 2-3 time until you stop seeing any significant solids
You don’t have to see completely clear water. But good to see few if any solids.
- drain your galley tank and then your grey tank to flush your sewer hose.
DON’t go off and leave the flush water running as you might get distracted and forget.

Word of caution. There is a black tank flush anti siphon vacuum breaker valve in the flush line somewhere in your trailer. Maybe under bathroom vanity. Maybe inside a wall. These are notorious for leaking water inside the trailer. If you do a search for black tank flush you should fine multiple discussions on it and how different members have either replaced it with a better valve, or eliminated it altogether.

Oops! My slow typing repeated what others said above.
Bill & Patricia
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Old 05-21-2022, 01:06 PM   #8
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You should add your truck andMontana year and model number to your signature. Then we can better help you as models are built slightly differently.
Also, maybe you figured it out, there are 3 or more tanks to drain - each with a separate drain handle. The one most people miss and have trouble finding is for the kitchen (in most units). The valve is operated thru a pull handle (metal rod) near or between the driver side wheels. You may have to get on the ground to look under there to find it.

My first time out, I couldn't figure something out and just walked down the rows of campers and found another Montana and asked for help!!

Tom and Gail
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Old 05-22-2022, 04:36 AM   #9
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One thing I do that was not mentioned is that I have an inline water meter that attaches to my hose. That way I know exactly how much water I'm putting into my tanks. This is much more accurate than just counting or timing the water input.
As BB-TX above said, drain the black tank until flow stops. Turn on the black tank flush water and watch for solids. When that appears somewhat clear, close the black tank flush valve and add about 10 to 15 gallons of water. Open black tank valve and drain. Knowing the size of your black tank will help as you can nearly fill it with water and flush it to get most all debris and "stuff" out. It should be around 50 gallons. I never add more than 40 to ours when flushing.
After flushing several times, I will close the black tank valve and add about 5 to 7 gallons of water to start the next cycle. I leave this in the tank, don't drain it. Go inside and add your chemicals to the tank for the next cycle. I use Unique RV Digest It, but any chemical you choose to use will need a good amount of water in the tank.
As stated, drain gray tanks to clean out sewer hose and you're done.
The meter is also handy for adding water to the fresh water tank using the "tank fill" option you mentioned above. I usually keep about 10 to 15 gallons in the fresh tank for use when travelling so we can flush the toilet and wash our hands, etc. It's never a good idea to travel with full tanks. Here's some links to what I use...
Robert & Diana McNeal
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Old 05-22-2022, 08:48 AM   #10
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An old fashioned tried and true method is to run a hose into your rig, drop into the toilet, close your blank tank valve, fill the black tank roughly half full, open the blank tank valve and let the hose run for 5-10 minutes until clear.
Mike and Lorraine
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2010 3665RE, 2015 3910FB
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Old 05-22-2022, 11:02 AM   #11
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We used to dump the black tank and close the valve and refill to the top of the tank with water and flush again ... Last time I did this with ours I went to fill the black tank with water and the water never came to the toilet.. Im filling the tank via the toilet flush... The Black tank On our 17 Montana is the 543 tank and it is two piece.. The top of the tank busted off and the sewage went everywhere...So. I suggest using the hose wash like Carl said..The water pressure from filling the tank broke the glued portion of the tank.. Notice the tape.. I guess the tape is supposed to keep the tank halves together...
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Old 05-25-2022, 01:40 PM   #12
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I also fill and flush per the procedure as RMcNeal in post #9 using an in-line P3 turbine meter to fill with the quantities noted.

I will add my hoses (city water and flush) are connected to the trailer AND shore water supply with quick disconnects. That way, once I charge the black tank with 15 gallons BEFORE use, I will disconnect that hose until I am ready to flush the black tank out, as a safeguard to mitigate the risk of the shore water valve leaking (have had that happen) or someone turns it on and inadvertently, both of which will overfill the black tank and onto the trailer floor.
2020 Montana 3780RL fifth wheel
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Old 05-25-2022, 02:21 PM   #13
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I personally don't bother with the black tank flush procedure. But yes to dumping black tank when near full.

Another trick, fill the tank 1/2 full then add a bag of ice right before you drive down the road. The movement will sloosh the ice around and it'll know any clingers off the sides of the tank.
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Old 05-25-2022, 02:44 PM   #14
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The black tank flush is a "feature" recently available. I had units since the 70s and many had not even thought of such a thing.
There were few problems if people used a lot of water in the toilet and didn't empty until over 1/2 full or heard the "burp".
At least one chemical supplier and many RVers say don't flush it out as then you flush all the good bacteria you have built up.
Me?? Now I flush in the spring when it's in storage for 3 - 3 months.
John & Patty
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Old 05-25-2022, 06:49 PM   #15
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CRITICAL! Dump Valve out=dump valve OPEN!

So, I had not RV'd for over 25 years when we bought our MHC in 2019. I also got a quick 10 minute walk-through, the dealer rep's first ever walk through. He casually waved at the pulled-out dump valves as he walked by and said, "There's your dump valves" and kept on walking. Now fast-forward to my first week of heavy use, and I go to hook up the waste dump hose for the first time. As I take off the waste dump cap, I am inundated by a tsunami of mixed black water matter, toilet paper, and two tanks' worth of gray water. The dealer had left all the dump valves in the open position, and didn't bother to tell me if out was open or closed--no labels! I learned the hard way "out" was open. I had to wade back into the flood to put the cap back on, then, between spitting and cussing, try to figure out what to do next. I finally shut all the dump valves, but still had about fifteen feet of four-inch waste line full of all sorts of nasty stuff, which now I had to deal with. I got a large storage tub, poured in a bunch of Mr Clean, turned my face away and opened it again. After about five gallons or so, the flood slowed, and I could hook up the waste line and reopen the dump valves, black then gray then gray. I then poured Mr Clean all over the spill area and washed it down with the hose, then cleaned the tub which I had dumped into the ground sewer drain. Lastly, I had my wife hose me off from head to toe, and threw all my clothes and leather sandals in the dumpster... Lesson learned the hard way.
Oh, yeah, you gotta see that s Robin Williams' movie "RV" for a similar scene.
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Old 05-25-2022, 07:13 PM   #16
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YouTube has great videos on RV do's and Don'ts but this forum has a wealth of info on how things work . Ask away.
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