I've never cleaned the roof of my fifth wheel, fearing I'd likely do more damage than good. I can see that a clean white roof might resist sunlight more effectively, but other than that, what is the ...
Keeping your roof clean is the best way to prevent black streaks down the side of the RV.
I also used a roof cleaner and treatment that supposedly had UV protectant in it on my old 5th wheel. We just traded it this summer for a new 5th wheel and the roof on the old one looked great. The old 5er was almost 14 years old.