How can I clean black streaks off my van????

(Moved from Social Room)

HELP.... I cannot get the black streaks off my van (motorhome)... it doesn't matter how much i wash and clean my van there are black streaks running down from  the external doors, which cannot be removed. I have been told that these streaks are coming from the rubber seals around the external compartments...how can i remove these, i have tried, all the normal cleaners, and Muc off..No joy???