We use spic and span liquid one a year with a soft brush.
When it is all dry we use a rubber roof protectant that you can get at most RV dealers.
When you wash it, immediately after rinsing it you'll have to wash the whole RV or you'll end up with black streaks from the roof rinsing, all over the sides.
We set aside one day a year for doing ours, and then a day or two later I wax the whole trailer sides.
Others might tell you different, but we have found over the years that doing it not only preserves the roof, but prevents black streaks from forming on the sides of the trailer.
Jack L