Again, the roof "supports" seen in the image from Winnebago (at the green X's) are nothing you could screw into. They are simple, flat sheet steel. Not I, H, C, or L shaped, they are flat, and sit vertically. They support nothing, the strength of the roof is in the laminated sandwich of foam and plywood. The metal bows are shown in detail in the prints Winnebago will send you. I can see one of mine above one of the lights in my bedroom.
And as far as screwing or gluing to the fiberglass, it's a very thin sheet, less than 0.040". It's not bonded to the 1/8" plywood underneath it, just laying there. It's only held down at the edges. (Search for Winnebago roof blowing off and you'll find a lot of info.) That's one of the reasons why I elected to bolt my antenna through the roof.