I'd be hesitant to use a power washer on the rubber roof. I've seen my preshure washer peel wood from my deck if I'm not careful!
As stated, a good non-citrus, non-petroleum based cleaner along with a medium/stiff bristled brush and some good ole elbow grease.
That being said, I've yet to wash the Diflex II roof on my 2016 and haven't decided what detergent I'm going use! LOL!
2016 Jay Flight 38FDDS