I'll stick to my story: they are a sandwich, not just fiberglass structure, and the interior paneling is put in in the mold. Take a look at
this recent article in Truck Camper Magazine, in particular scroll down to the photo of the interior paneling being installed in the mold. Also several photos of top and bottom halves being pulled from the mold, with paneling already installed. When I ordered mine from the factory in 1996 I was specifically told this is how it was done and it appears still is. They use a contact adhesive and pound it with rubber mallets to ensure contact. You cannot do that out of the mold. There are other pictures showing them assembling top and bottom with paneling in place.
If you think the fiberglass is the only structure, pull the paneling, foam, and wood stiffening out of the roof of one and walk across it. I guarantee you will think again.
From TC Magazine:
