Welcome to the wonderful world of stupid RV engineering! In a misguided effort to seal up the bottom of the coach, Fleetwood decided to cover the steel framework with a sheet of aluminum. As first year engineers know, steel and aluminum don't play well together and the aluminum sheet corrodes away, leaving the steel frame exposed to the elements and the reaction of all American Coach motorhomes with that design is that the steel framework rusts away. The worst spots typically are the compartments immediately in front of the rear wheels but all of the compartments suffer the same fate.
One of the most difficult problems is caused by the actual construction of the walls and floors. There is a sandwich of a 1/8" plastic layer, a layer of plywood, and a foam core. There are also steel re-enforcing bars buried in the foam which also rot away. I have never been able to find out what proceedure AC used to make the floors and walls, and also haven't figured out a good replacement structure.
The suggestion to join the Yahoo AC group is a good one. There are several on there who have done this repair and have offered suggestions. Good luck.
