They all leak!
If you open the rear hatch and get some light in there, you can get a pretty good look at the subfloor beneath the toilet/tub/sink area. Use a icepick or a screwdriver and poke the subfloor to see just how solid it is. Most, but not all, subfloors can have some amount of water-damaged area which usually means rusty frame underneath. That's to be expected in these beauties and is fixable, but it also will require some interior removal, etc.
Lou Reed said it when he sang "You need a busload of faith to get by." He could have added "You also need a coupla quarts of POR 15."
Best wishes and keep us updated on your adventure.