We have a 1990 Roadtrek that had a hard life thru multiple owners. The roof in many locations was beige (right down to fiberglass) and covered with mildew. We likewise were looking for a solution that would reduce heat in the summer. Here is our DIY solution that cost less than $200 ...
(1) Scrub the roof with TSP. Get *real* TSP (not the "TSP substitute" being marketed some places now) or use your preferred heavy-duty cleaner. You may need to lightly sand the roof, then wash again.
(2) Clean roof with "Dicor Fiberglass Clean & Prep" (
https://www.amazon.com/gp/product/B01B3P5EC4/) per instructions and be sure to rinse *very* well.
(3) Paint the roof with "Dicor RPFRC1 Fiberglass RV Roof Coating" (
https://www.amazon.com/gp/product/B007TQI96K/). You do NOT want to use the Dicor for Rubber Roofs version -- be sure to get FIBERGLASS Roof coating.
In our case we didn't care if we covered over the pin-striping or branding. The van itself had been painted by a previous owner and covering over the decals made us more stealthy, which helps in areas where "RVs" are not allowed.
Carefully mask with blue painters tape and LOTS of plastic. Anything that gets painted will stay painted, so mask carefully. Our three roof windows had horribly ugly black gunk that someone used to seal them -- we elected to paint over that rubber gunk with the Dicor white paint and it worked well. Or mask off the rubber gaskets. You either want to paint in the shade on a really calm day, or preferably inside.
Using a regular medium-nap roller (extension handle is a bonus) apply a generous, but well-rolled coat. You will need two, and possibly three, coats. The first coat will not look very good, but don't be discouraged, because when you are finished you will have an incredibly tough and weather resistant covering. It will seal all cracks and small imperfections, and is a *very* bright white.
When removing the blue painters tape, you may want to use a razor knife to cut the paint along the tape edge -- the paint is very tough and doesn't want to tear along the tape edge.
That's it -- a beautiful DIY new roof. We've got four years on the covering and its holding up very well. Looking a bit dirty, but otherwise very functional.