Bill,
Interesting suggestion, but I don't think it would be practical for a couple of reasons. Firstly, neither comes in white, which is what we would want to reflect heat and secondly, being a closed chain polymer, you would have a heck of a time getting caulk to seal properly around any openings. An asphalt-based caulk might work, but they're really messy and collect a lot of road dust, etc. Another consideration may be the additional weight of those materials and their relative inflexibility, which could lead to leaks and seperation of the underlying rubber membrane from the coach roof. Don't think I'd care to chance it.