Looks like the OP is doing some nice work.
On my 2001 Lance, I rebuilt the wings using marine grade hard woods, Coosa board for the inside, a rot proof fiberglass impregnated foam, used in boat decks, and 8 ten oz tubes of 3M 5200 adhesive and all stainless steel fasteners. I only wanted to have to do this job one time. On the marine plywood I used in places, it was encapsulated in epoxy resin on all edges and both sides. But in this part of Florida I have access to many boat supply businesses.
On my Lance, the underside was not caulked from the factory as best I could tell and I failed to spot this in my yearly maintenance. The rear tires drove the water into all the underneath seams when driving in the rain. What I thought was going to be a day or twos work, turned into about 80 hours of work from start to finish.
http://www.pajbcooper.com/Lance%20Camper%20845%20Repair.htm