They are streaks of dirt left behind when dirty water runs down the sides of the coach and then evaporates. The dirt comes from the roof, which collects a lot from various sources (dirt/smoke in the air, tree sap and drippings, leaves & debris, road dirt (oils).
Black streaks are a major problem on rigs with porous rubber roofs, because the dirt builds up in the pores and then runs off every time it rains, dews or if a/c condensation pools on the roof. Metal roofs can have a similar problem with dirt in the seams, but fiberglass and hard-finish EDPM (e.g. Britek) does not generate much in the way of black streaks.