To answer your specific questions: Every RV is going to have a framework of some sort, either metal or wood. A properly designed wood framework is perfectly acceptable if it is kept from rotting.
The cabover on any class C is not designed for occupancy while in motion. There are a few reasons for this. First, it is a very vulnerable location in the event of a crash. While crashworthiness is not a strong point of motorhomes in general, the spot exposed at the top of the front with nothing securing one in is worse than pretty much anywhere else in the RV. Second, the framework of a cabover is designed to support the weight of a person when the RV is not bouncing around on the road, but not the much higher dynamic loads that having the weight there would impose when going over a large bump.
Most fiberglass roofs are basically flat sheets of fibreglass set over a substrate of plywood, basically like a rubber or TPO roof made of fiberglass. The details of how this is attached to the tops of the walls vary somewhat. Very few class C's have a molded fiberglass roof like a Scamp trailer.
A rubber roof can last many years with proper maintenance (far more than ten years in many areas). A TPO roof is more durable still, and fiberglass properly built better yet. Any one will require proper maintenance and keeping up with the sealant aging at seams and penetrations.
(You aren't going to find any pre-2013 Transit based RVs in the US, of course, since Ford didn't sell the vehicle here until then.)
