I would never recommend anybody use a pressure washer in your roof. But I use a pressure washer with good results and have for many years now. But first a few cautions like anything else.
I have a max 2500 psi that I can turn down the pressure. I use the biggest nozzle I have which is a 40 degree. I won't let the nozzle below my knee level, so nozzle does blast a hole in roof. I don't use soap when using pressure washer, to slippery. I also got tired of inspecting the chalking around all the vents wondering if I missed any little tiny spot where water can get in. So every year I put a coat of rc2000 on the roof. Takes me about hour or hour 1/2. When putting it on I can tell exactly what and where I've done and where I've missed, because its wet looking. That way no surprises with leaks if I've missed a spot. And it turns out great. Just brushes on but takes forever to dry but looks good when done. That is in the spring. (Just finished yesterday, I'll take a pic if I remember) and in the fall I do my soap thing but I Don't get on top of roof when using soap.
Check out inland coatings and some of their "case studies" Surprise for many but RV's aren't the only thing with rubber roofs. Check out some of their case studies
http://www.inlandcoatings.com/casestudies.html (For fall) If you do use any soap be sure to use something that won't be to strong. I use Dawn maybe a 1/2 cup and put in a garden sprayer, then working with ladder because I don't want to slip and fall. I move around the RV with sprayer in hand and soap down the awning and the roof. After about 20 min I hose it off. I use just enough soap so I can tell where I've sprayed by looking for soap bubbles.
Guess what...I'm doing everything wrong according to the maint guides for EPDM and 99% of people on here. But I don't have any more problems like I used to with neighbors trees and very little mold. And I know I don't have any leaks when I get done.
Takes me longer to break out the ladder, take off vent covers and set up pressure washer than anything else.
(In pic vent covers are still off)
I also bought a 10 foot section of vinyl hose to put over the black pressure water hose because when you pull it around the edge of RV my hose used to leave black marks. Not anymore with clear vinyl tubing around hose.
Forgive all the RV's in back yard. Only some of them are ours.
Lets see, to many RV's
Wife Arctic Fox, Friends parking their pop up while they are in Alaska, Tow vehicle for wives Arctic Fox, Friends very good looking new Dodge 1500 for their pop up, and my great traveling machine "The Mighty Majestic".
PM me if you need more info
2003 Majestic 23P... Northwest travel machine
2013 Arctic Fox 25W... Wife "doll house" for longer snowbird trips
2001 "The Mighty Dodge"... tow vehicle for "doll house"