RVs with rubber roofs may get white streaks caused by the natural oxidation of the rubber roofing material. You can minimize white streaks by washing the roof twice a year.
Keep in mind when you are washing a rubber roof that the oxidation on the surface acts as a barrier to further oxidation of the rubber below so your goal is not to eliminate all the oxidized material but only to remove the chalk or powder buildup that will cause the white streaks down the sides of your RV.
To maximize the life of your roof use a soft brush (the same one you use to wash the rest of the RV should do) and avoid excessive scrubbing.
RV Repair and Maintenance Manual – Updated and Expanded The most popular resource for RVers who prefer to work on their own RVs. Featuring step-by-step procedures for maintaining and repairing RVs,