Our '92 Safari Trek had a fiberglass roof that consisted of a large flexible, thin sheet that was secured by the end caps and edge trim. It was trouble free, although we painted it after 10 years to cover some exposed fiberglass fibers. Never any leaks through the fiberglass.
Our current Beaver Monterey DP has a one piece fiberglass top that appears to be rather thick fiberglass layup with a gel-coat finish surface. IMHO this is the better of the two.