Hi guys and girls, first time here- and what a huge forum!!!

I'm after some tech advice (hopefully I've posted in the right section). I'm about to start building a camper trailer/tiny caravan of sorts.
The original plan was to design and build a teardrop camper, but then I started to add and remove features that I wanted in mine, and the end result is something that doesn't look like a teardrop any more, although it is a similar size. Many teardrop designs simply use thick plywood slab sides but I intend to frame mine up properly with a skin inside and out, and insulate it with some f/glass batts I have. I also intend to sheath my little camper in fiberglass to ensure its strength, durability, longevity and watertightness. Similar to boat building in that respect.

So, there's two things I want to know:

What is the best material for my framework? The lightweight camper framework will have 'studs' at 450mm centers which will accommodate the fiberglass batts. But I hope to keep it as light as is practical. I would need to get my local cabinet maker to supply as I don't have the facilities to rip it down myself. What timber would fit this bill- light(ish), strong, commonly available and economical?

Next is the skinning of the little camper (one for the boat builders here). Being completely sheathed in fiberglass, the marine plywood external skin won't need to be real thick, but how thick should it be? 4mm? 8mm? Would I get a superior result from 2 layers of 4mm instead of one layer of 8mm? or am I completely off the mark?? Maybe 2mm or 18mm?? Any suggestions?