DO NOT use a pressure washer on your rubber roof, even though it appears to be effective, a pressure washer removes some of the top layer of the EDPM rubber which opens its pores and it then becomes even more difficult to clean.
Most liquid detergent products should clean the dirt and grease related soiling, but if the black is not being removed, use a bleach solution and rub with a fairly stiff brush.
While using any cleaner, be sure to have a hose with good water volume and it helps if you have someone on the ground with a second hose to rinse the residue all the way off the coach.
Without the second person on the ground, your coach will be streaked with a milky residue, which is in itself difficult to remove.
Once you're done you might want to try one of the rubber roof sealant products, though I have not been impressed with their ability to resist soiling.
While you're on your roof, it's a good time to inspect all your seals and repair any cracked seals with self leveling roof sealant, which you can get at an RV supply store.
Steve gives good advice - I second every bit of it.
You don't need any of the "rubber roof cleaner" products. Regular detergents and a bit of bleach works fine. I haven't found that any of the rubber roof treatments products (that are supposed to keep them shiny and reduce cleaning time/frequency) to be valuable either.