The backs falling off!

On my last TT, the back end was rotting out badly and the floor sinking.

Black tank fell out in the highway once. By some miracle it held on one side and the end that fell out hung up on the stabilizer jack. That's the only thing that stopped it from hitting the highway. That would have been a mess.

I temporarily rigged it back in place to get home and came up with a simple solution to affix it there long term without a lot of effort.

I simply got a piece of angle iron to go across the frame rails and I metal lag bolted it up into the bottom of the frame. The angle iron was tight up to the bottom of the tank. I first used a board and a bottle jack to push the tank up into place.

Took about 2 hours and was very effective.

I have decided that I am going to drop the tanks, fabricate a cradle for them that can be fastened to the frame extensions. I am then going to shim up the rear end floor and just pray it doesn't get too weak over time. This thing is over 30 years old but I am hoping I can get another decade out of it. Obviously this is going to be a long project and I'm not sure when I can get it done.

Did you manage to come to any conclusion about the problem?

Wood framed floor?

Chances are the floor is rotting out inside and settling down on the frame. It is sinking in the back at the rearmost frame extensions.
Same principle as other RV construction.

On TT's with the same issue you can tell when the floor is rotting by looking at the very back outside and see if the siding is denting into the top of the main frame. If yes this means a rotten sinking floor. Or in your case the fibreglass.

Another telltale sign is the frame to floor bolts at the frame outriggers. When the floor rots out the carriage bolts hang down like a pendulum.

Also, reach under with your hand and grab the wood floor outside rails at the back. Grab them through the underbelly skin cover and see if they are soft and mushy.

The crack in the fibreglass is a telltale sign too.

Also, watch the tanks don't fall out. When bolted to the floor and the floor goes away, there is nothing holding the straps. They give way and you lose a tank! Ask me how I know this!! :E Been there done that!

Check these items out to see. If yes to any of the above your floor is rotting out from undetected water intrusion. This is very common.

I think its best if the tanks are strapped to the frame ,not the floor.

I have looked at the frame extensions and they look fine. No signs of cracks I am thinking the failure is in the plywood floor/undercarriage. It may have some water damage. Anybody heard of someone moving their tanks so as to put less strain on the wood?

I would say check all the rear roof chalking seals first. If they are good then go to the frame.

Definitely check the frame extensions!

Check your frame. Most motorhomes have frame extensions and maybe yours is having issues.

Rear leaf springs.
Need to be replaced or add one leaf to assist the old ones that have fatigued.