Hello,
I'm replacing sections of the rotted subfloor of an Airstream (a problem common to almost all RVs), and I thought I'd share a few things that I learned from this job and also from building my woodframed foamie, the Nutmeg.
After removing the rotted plywood in the Airstream, I found sopping wet fiberglass batts in the underbelly, complete with snails and wasp nests. After excavating this nastiness, I found that the frame was not seriously rusted. After knocking off the loose rust and scale with a wire brush, I painted it with a paint that transforms the rust permanently to a polymer. There are several brands - "Paint Over Rust", "Chassis Saver", and they cost about $50-75/quart, and there are other treatments that also convert rust. Note that these products work best with rust, not bare steel. For those who build their chassis from scratch this might be a good option.
After painting the frame, then I cut EPS to fit between the frame beams and glued them in with Gorilla glue. It's probably a good idea to punch a few drain holes to allow any seepage to escape. The beams of an Airstream are 5-1/2" deep, so there would be a 3-4" airspace between the EPS and the aluminum bellypan (hmm, do any tiny trailer builder install bellypans, and would that have any advantages?
Recently, I have discovered a treatment used for log homes to prevent/treat insect damage and fungal rotting. One product is called Timbor, and here is a link to a discussion on an Airstream forum. It can be applied before or after installing the wood, so long as it hasn't been painted.
http://www.airforums.com/forums/f36/bor ... 84474.html