Not to recreate the wheel, here are a few replies I've saved for these times. (Disregard nonapplicable data)
(too late, but still informative)
Overlander Bath Floor Repair Thread
You need remove some skin (aluminimum, not your own, although you may lose some flesh as well) to see what's going on underneath.
The aluminum lasts forever but the steel frame is prone to rust and the plywood floor is prone to rot. Suggest you do a search on rear end separation.
Most newbies are hoping to have a usable unit for $5K-$6K. I've found that to be unrealistic in my experience. Bottom line, you need to make a thorough inspection to determine the source of water causing the rust, which is either inside plumbing, or outside leaks. You need to determine the source as well as inspect other components (rear floor/rear channel joint) to be sure you're good to tow. The black water tank supports in the rear have a tendency to rust out as well. (Axles are a whole 'nother subject!)
I've had a little bit of experience in working on these things ('72 Overlander, '73 Sovereign) so I may seem a bit jaded. (I took lots of pictures and posted them in the photos section)