My wife and I bought a 1985 Eldorado Prelude Class C last month and are facing the same situation you are. I've been in the trade show exhibit business for about 25 years and your post got me thinking about materials we've used on exhibits.
Sintra (Sintra PVC
) panels come in a variety of sizes and thicknesses (3mm is about 1/8"), weighs very little, is flexible, and water proof.
Another nice thing about Sintra (or any of the PVC sheets) is that it can be bent with a hot wire. It can also be permanently bonded to other pieces of PVC sheet with MEK (Methyl Ethyl Ketone) but I'm not sure that would be useful for our purposes.
Sintra (and similar products) can be purchased at sign supply stores and purchased in single sheets. I have access to a bunch of old graphic panels so I'll let you know if I figure out if it will do the trick.