Plumbing leak

[English Lake Montana]

I have a plumbing leak issue with my 2007 Montana 3400RL I need help with. I was flushing out the black water tank with several 3 gallon pails of water because I could not get the light on the tank level to go below two thirds. I have had this issue for a couple of weeks now. After dumping the water down the toilet I checked the reading and it showed the tank to be full. I then opened the black water drain valve and while the tank was draining dumped three more buckets of water down the toilet. The tank reading then dropped to one third full. I then closed the black water drain valve and then drained the grey water tank to help flush out the sewer line.
I then noticed water trickling down under the camper on both sides right behind the rear axle through a seam in the black material that covers the bottom of the camper. The water looked fairly clear but had a slight sewer smell to it. When I pushed up on the sagging black material in the middle it started pouring out from other gaps in the outer edges near the axles. I somehow managed to stay dry. I would say about a couple gallons leaked onto the cement. I left a board underneath to hold up on the middle of the sagging area to help it dry out. The areas in the front and the rear of the camper stayed dry and as far as I could tell had no water collecting above the black material. Where are the black and gray water tanks located in a 3400RL. Is one located near the rear axles where the water collected. Its hard to tell which one the water came out of by looking at the color of the water because of the large amount of fresh water I dumped down the toilet before draining the tank.
One more note. We just purchased the used Montana and have used it three weekends. It is parked on a permanent spot on a lake I own some frontage on. I have a well and septic system it is hooked up to full time. I normally do not close the gray water valve except when draining the black water tank. This last weekend before the leak occurred, I had decided to keep both tanks closed and use the gray water to flush out the mainline after dumping the black tank so it could have been either the gray tank or the black tank leaking. Any tips on how I could trouble shoot the problem. I am probably going to have to go underneath and remove a section of the black material and visually inspect for leaks. Any help would be appreciated. Thanks.

[bigskyjimmy]

It is not hard to remove the underbelly material just time consuming, remove all the screws on the sides remove the support braces and it all comes down with the foil insulation behind it after you do that take a look and see where it is coming from ,I have a 2012 3402rl same basic floor plan as yours and my main grey tank and black tank are side by side(Grey is driver side black is pass side) under the front belly cover under the hallway/bedroom area the Kitchen holding tank is located well under the kitchen belly pan get a good flashlight to get a good look ,maybe you split a seam or a valve broke loose but you will see

[8.1al]

Just in case you weren't aware of it you have two grey tanks. One for the galley and one for the shower/sink.

[Montana_9856]

This topic is a duplicate. Please post a question only once to ensure all answers are in one location. The original posting is here:
http://www.montanaowners.com/forums/...ad.php?t=59115