Apply gutters on your RV. This will prevent spots from appearing on the fiberglass roof by capturing excess moisture.
Place an RV cover over the RV when it sits for an extended period of time. This will protect it from precipitation and pollution, ultimately preventing black streaks or spots.
Sweep the roof often. Wait for the area to be completely dry, and use a soft-bristle broom and dust pan, and bag it up right away so it doesn't get pushed down the side of the RV.
Wash your RV every other month with non-petroleum based soap, and wax it, too.
Check your RV often for black spots and streaks and maintain the roof. Regular maintenance will help to keep black spots under control, but nothing will completely stop them.
