Sagging wall and soft floor are both likely caused by (a lot of) water intrusion, assuming the wall is wood framed. Wall repair is conceptually akin to floor repair, just vertically rather than horizontally: remove all the rotten skin, framing members, etc., and replace. It's not a quick Saturday afternoon job!
Basically, it sounds like you'll end up rebuilding half a camper...one with a frame that may be questionable. I'd think long and hard about cutting my losses before diving in; but on the other had, if it's the kind of work you enjoy doing as a hobby, perhaps it's worth the effort. It might not be much harder, though, to get a box truck or a step van or a bus or something and build up a new RV in that, reusing appliances and fixtures scrounged from the trailer.
