After several request for further info, I am posting a few diagrams on the construction.
These were created in Rhino3D which is a CAD program. I use this program in one of the classes I teach at high school :)
On my Fourwinds Class C RV, the frame work is aluminum. The whole cab over drawing has been simplified, but is fairly accurate.
Each section of the frame is in a different color
Roof - Rust 2"x 1"
Cyan - Sides 1"x 1"
Purple - Front Window Section 1 x 1
Green - Bottom Section 1 x 1
The sandwich is constructed of plywood and foam board. With the plywood covering the aluminum frame. Either 1" high density Styrofoam board (Blueboard foam) was used or 2" in the roof. I used 5/32" exterior Luan Plywood from Home Depot. I believe the actual material used in construction was thinner (1/4").
The ceiling/roof actually has a surprising amount of strength from the 2" thick sandwich. It can support the weight of walking on it.