I have a class A with a fiberglass roof. I park it along a tree line in the summer & inside in the winter The roof is black on about 25% of it. I got up there today with a bucket of oxyclean mixed ...
I would not use a pressure washer. (yes, I know some do...) Can you see the roof from below? If not then just leave it alone. Doesn't hurt it at all to be dirty.