The nature of the black streak varies by the roof material and the dirt & chemicals that collected there and ran down the sides. Sometimes they can be really difficult to remove because they etch into the surface.
RV stores have "black streak cleaners", and sometimes they work well. Other times, not at all. Some car or boat detailing sprays may work as well. If they don't work, you may have to use a vehicle or boat polish to rub them out.
Streaks are usually easier to get off painted RV surfaces than plain fiberglass or filon sidewalls.
Former owner of 2004 American Tradition and several other RVs
Home is in the Ocala Nat'l Forest near Ocala, FL
Summers in Black Mountain, NC