There are a few RVs including TTs that have fiberglass roofs. In my opinion, they're not a great idea. The whole trailer flexes as it goes down the road. Something as rigid as fiberglass is going to need very flexible seals and is going to need to be attached with something flexible... or the structure of the entire unit needs to be very rigid so no flexing occurs... otherwise it will destroy itself (ie seams pulling apart or breaking in the weaker areas). I believe the Aviator that Forest River tried has a FG roof... but I may be wrong about that.
Jack and Dee Dee Weatherford, Texas
2014 Ram 2500 Crew Cab 4X4 w/CTD 6.7
2016 Jayco White Hawk 28DSBH Travel Trailer