I'm not a great fan of using pressure washers on the motorhome, especially the roof.
The coach we just got also had some sort of sticky little round deposits on the fiberglass roof. I could not really scrub them off. So, I had one of those metal floor scrapers on a long handle (like you would use to scrape chewing gum off a floor). I used Turtle Wax Zip Wash. I wet the roof, used the scraper to loosen the deposits, then scrubbed the area with a long handled stiff brush. It cleaned up really well.
I use 303 Protectant on the roof after washing it.