Treat the fiberglass roof as you would the sides. Myself on BT cruiser I bought used, had a dirty fiberglass roof more dull with oxidation and dirt that settled on oxidation 9 years worth. I never thought it would be able to be removed, but I am a clean and shine nut so I put the time into it, lots of time
. I used light scrub brush good dish soap and depending on how chalky a lot of time and elbow grease. The entire roof took me about 2-3 hours and came up close to new again, I was very suprised how the roof came back like new. Once after I rinsed off roof and it dried. I wanted to get the lap caulking white to, I use on a cloth lacquire thinner and it removes years of dirt and grime from white caulking on roof and restores it back to bright white with out harming caulking. I followed up with Protect all Wax with UV and waxed then buffed cleaned roof. now like the sides a good wash with car soap and rinse keeps roof shiny new. Like the rest of the coach before storing BT in garage for off season she gets wash and wax from roof to tires. I also when I bought coach used high speed compound wheel with proper pad and McGuires compound for fiberglass and totally restored fiberglass sides, BT also came back to new. My Bt looks brand new now, it was a lot of work, A LOT!, but once you get it back it will stay new and never have to work hard to keep it that way again, if you wash and wax yearly and keep it covered.
I also find protect-all a great wax and cleaner to use on the RV really does a great job protecting and keeping Fiberglass shiny and the UV inhibitors keep the sun from oxidizing. Hope this helps others with keeping roof clean, its important if you want to keep your investment tip top!