As Larry said, black streaks are a combination of dirt and chalk from the roof, basically dirty water than ran down and left a residue behind. It is particularly a problem with EPDM rubber roofs that have a chalky residue that carries dirt well and makes for a nice adhesive. Wash the roof regularly to minimize the dirt build-up and apply a liquid floor wax or "RV rubber roof treatment" to help reduce the chalking.
A good coat of wax on the RV sidewalls helps as well, but the dirty water runoff can and does still stain the wax. However, better to have the stain on the wax than embedded in the fiberglass or painted sides.
As far as cleaning off the streaks, the composition of the black streaks depends on the type of dirt that collected on the roof. The dirt from a forest is different than industrial dirt from smokestacks or acid rain. Different cleaners yield different results depending on nature of the dirtiness. A cleanser that works easily on one may fail utterly on a different type of dirt, so experiment with different cleaners and methods. Sometimes only car/boar rubbing compounds will work.
