Here is what I believe a cross section of the door would look like:
FWIW, I think the door skin is as Wolf Whistle said, "constructed with 1/16" fiberglass bonded to about 1/4" (7/64"?) Plywood." The skin is glued to the foam to make a sandwich (I like the sandwich analogy better than the I-beam analogy). The door sandwich is lined around the edge with the aluminum extrusion. Per the Lippert Engineer, the aluminum extrusions are joined at the corners with a "glued in key".
I don't have an issue with the door construction. My problems are the way the door is hinged and the springs used at the hinge line to make the door easier to raise and lower.
Using springs at the bottom of the door exerts tremendous pressure on a small section of the door. The pressure is enough to distort the door when it is new. The distortion gets worse with use.
Contributing to the problem is the fact that the hinges are very short.