Lots of opinions... Here's one more!
I have a 2004 3295RK and I have replaced 2 of the tanks and repaired the galley tank so I am very familiar with how they are built, at least in my rig. (Sometime in it's past, previous owner... it froze with full tanks and broke the top right out of 2 of them.)
The only reason I mention the fact that I am familiar with the tanks is because I have experience with how mine are plumbed. The drop from the toilet goes through the basement and into the top of the black tank very close to the tank outlet (Deep end of tank) That's a good thing! Imagine starting out with an empty tank and dropping a couple of heavy loads down that 3" pipe... SPLAT! and the mountain starts building.
I feel it's important to put a minimum of 5 gallons of water (35 gallon tank in my case) in the black tank and whatever chemical you prefer before putting the tank into service. This allows the floaters to float and begin the decomposition process like they would in a septic tank. Sorry if I'm being too graphic! As stated earlier in this thread... use plenty of water when flushing. Pretty hard to fill a black tank.
I digress to flushing... When I started with my new tanks I put in one of those tornado flushers. I can hear it spinning when I hook up to that line, but I am not really sure how much good it does in there. I use the same connection to refill (recharge) the tank for it's next use, or when I am going to travel for any distance. I shoot for 5 gallons, but I am not sure how much I actually get in there. The water sloshing around in the tank while on the road is great for stirring things up, and really works great if you can dump the tank again before anything can settle out. I like the idea of laundry detergent in the tank before travel. Might have to try that sometime.
On probes... as others have said the OEM probes are worthless from about day 2! When I installed the 2 new tanks, and repaired the third tank (Galley) I installed
Horst Miracle Probes in all 3 of them. They seem to work as advertised. I get good readings, even on the black tank after about 5 or 6 cycles since the install. They are easy to retrofit if you can get at your existing factory probes.
At the end of the season, I have a home built device on the end of a stiffer piece of hose that I shove down the toilet and work it back and forth with hot water through the garden hose to give the black tank a good rinse before putting the rig to bed for the winter.
In closing, when testing these tanks for leaks before I buttoned the belly up again, I filled them full. If you ever saw the shape a 35 gallon ABS tank takes when it is full, you would never forget to empty them before you hit the road! I was shocked at how distorted the bottom of the tank gets with all that water weight in the tank. I suppose the belly plastic on my rig helps support it, but I will never move with anything over 1/2 tank full after seeing that!
Sorry for rambling!