They are not lightweight. In the case of many Class C motorhomes build on the Ford E chassis, the manufacturer has used plywood for everything. There are certainly opportunities to build with marginally more expensive materials to make major weight gains.

For the uninitiated, what materials are we talking about here?

The plywood mentioned by GP is kind of heavy, and it needs framing for stiffness too. The molded steel used in "real" cars and trucks would be lighter. So would something handmade from carbon. Just thinking by analogy with boat hulls, I don't see why fiberglass wouldn't work either. My parents have a pickup camper the walls of which are made from Styrofoam or something similar. This seems to work; I can climb on the roof to adjust the antenna and I'm fat.