I have an early 2001 Tiffin Allegro Bus and the roof is fiberglass. It is sealed with a white silicone sealer along the panel joints and very sturdy. I clean mine with Simple Green HD and (It is blue in color) and a small electric power washer I purchased at Home Depot. So far as I know mine is still sealed as it was from the factory and has never leaked. The white silicone they used at the factory is still pliable and will dent when pressed then return to position when released.
Hope this helps.
My wife and I would travel full time, but we have to sleep occasionally.
2001 Tiffin Allegro Bus 40 ft DP named the Wild Eagle
Followed by the Eaglet - A 2014 Jeep Grand Cherokee.