The basic idea behind a foamie is that there is minimal wood, to preclude rot, and that the strength of the wall and roof panels comes from the inner and outer skins which consist of glued on fabric, or "poor man's fiberglass". This can be likened to a SIP, or structural insulated panel.
The strength to resist bending in any structural member comes from the limits of the outer fibers and how far apart those members are placed. So by having relatively strong fibers in the outer layer, and keeping those fibers separated by a light weight, but fairly incompressible material, a very light and strong panel can be made.
For a much more technical explanation of bending resistance study up on
"area moment of inertia".
Some will say that a foamie must also have a certain simplistic and thrifty quality, which is definitely an option, but, IMO, like any other build technique, the method can be used in any shape or form from basic box to elaborate and sophisticated. It is up to the designer/builder to decide what is within their scope (budget, time allotment, and skill level) and how far they want to take it.
There have been very simple box like builds with no galley or hatch; traditionally shaped TD's with hatch and two doors; rear door standies; canned ham shaped standies; and a couple of hybrids with thin wood paneled interiors. Although my own build is not complete, TPCE falls in this hybrid category and is well on its way.
Glue, canvas and paint has long been used as a deck covering on wooden boats, and several more traditional wooden TD campers have used this technique as an outer covering, but these should not be confused with a foamie. A "traditional" foamie has very little wood; perhaps just a simple plywood floor, some framing around any windows, the door and door opening to maintain durable and true surfaces for the seals; and perhaps at the hatch hinge, if equipped.
Now that said, we foamistas tend to be willing to experiment, to try new things and nontraditional approaches, so there really are no limitations; but the underlying unifier is the canvas over foam method. All other details or combinations of build methods are game.