I don't agree that the roof material is a problem. It is the roof seams that can be a problem, regardless of material. A rubber roof should not be a problem until it is old, or is torn by a branch or something. I'd rather have fiberglass, but there are bigger problems on Class C's.
Unique to the Class C, the truck camper, and to some extent a fiver is the bottom sidewall seam on the front overhang bodywork. There is a corner trim piece that wraps up onto the sidewall. Because the opening is upward, it can catch water. Over time the sealant can deteriorate.
We had an old fiver with several leaks. I realized that every corner trim that faces even partially upward is a problem. By partially, I mean it angles, like "\" or "<". So on those pieces, any portion that can see the sky can catch water. If/when a leak path develops, water gets into something. On a cheap C which doesn't have a molded front cap, the filon front end is shaped like a C. So the upward facing part of that C molding is another leak prospect.
You see a lot of leaks on C's that don't have front windows, and I think the two areas I just mentioned are the culprits that are unique to C's and TC's.
Another factor surely is the extra shaking/vibration of the overhang. Maybe worse on a TC than the other types.
Then there is the front cap, side windows, and marker lights. Those are no different than the same items on other style RVs though. Except for vibration as above.
