If this helps any, the rear wall of my 98 Starcraft TT was not built like the side walls. I had it apart after a tree fell on the rear of the trailer. The rear had the wooden joists and insulation, but no outer layer of luan (thin plywood). Instead, the fiberglass had some thick paper (think light cardboard or super heavy paper) glued to the fiberglass to stiffen it. I found this very strange but, thats how it was. So now you know how it goes together since I'm sure you're now extremely curious after finding many units are built like that. I'm not sure what the reason is for doing it that way, but it's apparently a design that's been used at least back to 1998.
1988 SunLite Hideaway 9.5' TC project
1996 Ram 3500 Cummins dually highly modified