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.