I have an old 1992 American Eagle, it is all undercoated. The only rust I have is where the water bay is, the white panel inside has rust under it, hubby is going to take it out, paint it and fix it. I suppose it has leaked around it, and the same underneath that area in the back corner.
The other is the propane tank needs wire brushed and repainted. I had it pressure checked and new type valve put on because folks weren't familiar with the old type and had trouble getting it filled.
I had a newer 2006 and that motorhome had a lot of rust, it had no undercoating which apparently makes a big difference.