Sorry, I have myself run into inconvenient facts many times over the years! I submit that it is difficult to beat certain minimums in the light trailer endeavor. Really, a wood frame and cover can be pretty darn light, see the Wanderer in the Design Library. Fast to build, also, comparatively.
There are some concerns with a folding design that need attention- the separate parts need to be rigid, resisting racking forces. A young fellow built a trailer with raising roof of foam and had trouble with joints after pulling it many miles over rough roads. Maybe a few 1x2 lap joints with epoxy fillets at the laps would provide lots of rigidity, and be pretty low tech, too.
For rot resistance, a little bit of cedar would maybe be a good choice? Just use in such a way that swelling and shrinking doesn't affect the look and function of exterior covering...on my trailer plywood used in window openings and glued in with some latex goo showed pretty sharp little bumps when humidity increased during rainy season. I have glued on aluminum strips (with polyurethane caulk) to cover those edges. Then put in the windows. I thought the surface looked like it might work enough to crack paint thus compromising waterproofing...My trailer is pretty light, minimum wood...
The upshot is, there are different ways to get there, the main thing is, don't overbuild. Nodrog