Use some dish soap and a soft brush. If the soap and brush do not remove all of the black streaks, there is a product made just for that by thetford. Be sure you have the time to wax it when you are done. The wax will prolong the time period before the streaks show up again. Since it is Automotive paint you can maintain it like you would your car. Once it is clean DO NOT use dish soap as it will strip the wax.
WBCCI #5501 AIR # 49
1978 Argosy 28 foot Motorhome
Wise men talk because they have something to say; fools, because they have to say something. -- Plato