Saying a Bigfoot is a wood camper is as incorrect as saying it is a fiberglass camper. The primary structure is a sandwich, composed of a ~1/8" fiberglass skin, bonded to a 1 - 1 1/2" foam core, which is bonded to ~1/8 luan plywood. Though the materials are different, this is similar in concept to cored boat construction. Each of the components individually are flexible and somewhat weak, but when bonded together they act essentially as an I beam with the skins replacing the flanges and the foam replacing the web of the beam. Remove any of the components - including the glue bond - and you are back to individually flexible pieces.
Anywhere there is significant load on a Bigfoot there is wood: around all the windows, hatches, and doors, anywhere tables or furniture is attached, backing for the jacks and hold downs. The pickup box sides are plywood embedded in the glass and floor is entirely wood. The sandwich construction makes a stiff panel, but will not take high local loads, or hold screws well, or deal with compressive clamping loads. That's why and where you put the wood.
Fiberglass isn't a light material, if you made a structural shell only from solid fiberglass it would weigh much more. You would need to double or triple the shell thickness (and still reinforce high load areas), that would add ~500 - 1000 lbs to the camper. The sandwich construction takes advantage of the foam (already needed for insulation) and interior paneling (already needed for finish) to accomplish the needed stiffness without that increase in weight, and wood is a weight efficient (and cost efficient) reinforcement for areas that need it. Unfortunately it will rot. Ideally fiberglass hat sections or other methods would replace the wood, or a real structural foam would replace the styrofoam. And you can get that in a $200K camper - but not $50K camper. Bahn for example actually does build a camper shell like a boat - fiberglass inside and outside skins with structural foam core. The bare shell starts at about $50K.
