"I use a pressure washer. There I've said it":)
Here is what I do and get good results. First ...think about your equipment before you get up there. I have a 2000 psi with a very wide nozzle. Nozzle is 45 or 60 degree. And I don't get it close to roof because I went to a place that repairs pressure washers and they made me a very short tube. When I hook up to the wand with this short tube, it only comes to my knees when hanging down. Think they charged around $20.00 for cutting to length and putting on a connector. That way there is no danger of blasting thru the roof with to much water pressure when I'm standing upright. I also picked up 10 feet of clear vinyl tubing (cheap) and have the last 10 feet of my hose slid into it, like a protective cover. No more black marks on roof edge or when I drag it around. Just lay out 10 feet of vinyl tubing and cut length wise. Open it up and slide pressure hose inside. Little bit of dish soap to make it slippery if hose binds up. Everything on top that is plastic I give even more space between the nozzle and point of contact, respecting what water pressure can do to plastic and skin. And the hose doesn't roll up worth a darn with vinyl tubing around it.
Other points to watch. Your balance!!!! When you pull the trigger!!!!!
I stand just about in the middle lengthwise (not on edge) when washing and I wear "deck" tennis shoes. It gets slippery depending on what type and if you use soap. Being my ladder is at the rear and right in the middle of RV I usually drag hose up RV ladder, so hose is in middle of RV roof. Hose just follows the top rails of roof ladder. I Don't move my feet unless I have checked where my feet will go first. Another big hazard learned first hand is... with a very short tube...don't pull the trigger if the water could get you. That "spa treatment" in exfoliating my skin doesn't take long. You'll know real quick if you shoot yourself with water. Stung for about an hour, and looked like a bad sun burn couple days, but lesson learned.
I never touch sides with pressure washer.
We've all heard stories about falling of the roof. Anytime your feet are not attached to the earth, there is a possibility that Mr gravity is going to get the best of you. I have actually meet a man that was washing his roof by using a ladder with a long handled brush and fell off the ladder when he was brushing away. He now travels 35 miles one way, to where he can park along side a flat roof and get his roof washed. Of course we all can't have a son with a flat roof on his garage:)
If roof is real dirty, I'm going to use lots of soap. I stand where I can more less stay in one spot. Saves excess moving around. I also rinse really, really good. Because any left over soap will be very slippery.
When washing RV the worst part to me is all the weeds that pop up.