Fiberglass roofs are gel coated, some are painted afterward to exactly match the rest of the Van.
The Gel coat is first sprayed into the waxed roof mold, then the fiberglass laid on top, and when popped from the mold you get a nice shiny fairly durable exterior like a boat hull.
There are products which mimic the gel coat and can be applied after the fact, but getting the same shine and durability is not easy.
https://www.google.com/search?q=spray+on+gel+coat+repair&oq=spray+on+gel+coat+repair&aqs=chrome.0.57.4581j0&sourceid=chrome&ie=UTF-8
Cracks in the gel coat will allow the slow degradation of the fiberglass below.
Polishing the cracked gelcoat will slow down the degradation, but pushing wax/polish deep into the cracks will repel later attempts at a correct repair, and the gel coat needs to be sanded away.
Regular polishing and waxing can prevent the cracks in the gel coat to begin with, but out of sight, out of mind.
My roof was all cracked when I got it a decade ago. I roughed up the surface with 100 grit, and applied Interlux premuim Yacht enamel with a roller. Proper prepwork is still paramount, and the paint is not a permanent solution. I've repainted it a few times, the latest with rustoleum yacht enamel which seems to be pretty durable so far. It appears letting it get and stay dirty is corrosive to the paint. I'm trying to polish the roof twice a year to help remove these etching contaminants, as well as allow them to slough off easier when it rains.
The results are nowhere near those of a new roof popped out of the mold, nor nowhere close to what a professional paint shop could achieve. I don't have the funds nor requirement for that level of finish.
