We had the drooping backside problem in our Lil' Bigfoot
. What caused ours was a wet and rotten floor that had come unglued from the underlying fiberglass (and of course the trailer stops a foot short of the egg's bottom; why, I don't know!). After we removed the old floor and recut the plywood we applied glue and then used a piece of plywood and 2 jacks to push the trailer bottom up against the new floor. We did it in 3 sections and left the jacks/plywood in place for 24 hours to allow the glue to cure.
Our floor was rotten because we had leaks
at all 4 windows
(working on those now), around the door, all around the bellyband, from every pipe perforation, and through the roof vent - hopefully yours will be a simpler problem to solve.