The rear wall is aluminum framed ... there is a 1/2" (maybe 3/8") sheet of particle board over the exterior. The sidewalls are vacubonded together with luan, insulation, and filon (fiberglass). They just hang a sheet of particle board on the rear wall for added structure as what used to be a standard Montana has the rear cap ... no filon skin. There is fiberglass batting (insulation) between the particle board and the cap.
It's an older brochure, but not much has changed stucture wise other than the roof rafters. Enlarge the frame photo on page 4 and you'll get a better idea of what I speak of.
http://www.rvcountry.com/IRV/Files/262789_1.pdf