My personal experience with a membrane roof is that the "dirt" for lack of a better word, is a combination of mildew/black particles and other forms of discoloration that are pretty well-fixed (almost embedded) to the rubber. The surface is not slick or glazed and whatever dirt is on it is attached pretty well. I think (I've not tried it) pressure washing would achieve some "change in color", but to get it really clean, and particularly if you are going to treat it afterwards with some recommended dressing, then it's gonna take that brush and a lot of elbow grease to get it clean. If a detergent or some form of soap can be infused into the pressure washer stream, then that might make a big difference.
If you choose to use a pressure washer, I'd advise being very careful how you "direct" the pressure stream onto the roof. RV vents do not "seal" air tight and depend on rain falling from above not to leak. A pressure stream directed close to the base of a vent is going to wash up and under the vent lid and fall inside. Same goes for refrigerator vent as well. Anything going into the vent stacks for grey and black water would just fall into the tanks with no harm done.