Black marks on vans

With all the previous talks about cleaning the black marks from their caravan or motorhome I have found the solution for myself. We are fortunate to keep it at home but on a slight slope so I bought a roof cover that was slightly bigger than needed and it folds about 1 foot down the sides which allows the water to run off the roof and miss the van. Has anyone else done the same, also with the long straps it is easy to pull over the van from the side without using steps. I did make sure that it was the breathable type.