There are campers made with aluminum studs,dont recal name right now,,do you know how to weld aluminum?
It takes expensive equipment...
Steel on the other hand is easy to work with..
I would never use 2x2 wood frame,too flimsy and weak..
I think it depends on what your design parameters are for your camper. Is it just shelter? Or do you want to be able to carry weight on top?
I'm currently building a camper with a steel subframe (16 gauge tubing) with ply skin sheathed in fiberglass, due to my own requirement that I be able to basically treat it as a contractor lumber/ladder rack if need be.
I think you could build a very strong camper using all wood construction...especially if (as was mentioned above) you borrow some pointers from the boat building world. Epoxy really is an incredible invention, and combined with wood (especially plywood) is a match made in heaven. Anyone who can mix creamer into their morning coffee and put butter on their bread can mix and apply epoxy. It requires no special skill or expensive equipment, unlike welding aluminum. And if you spend a little time familiarizing yourself with the right ways to laminate up wood plys, you can get a tremendously strong product that will absorb vibration and flex with little-to-no fatiguing. Several well-known sailing catamaran designers (including Richard Woods of Woods Designs) have great praise for plywood and epoxy construction for this very reason, considering that ocean-going sailing catamarans are under incredible flexing loads and need to be strong enough to handle them without failure, yet also must be light weight enough to enjoy the speed they're expected to achieve.