The only thing the pressure washer did was remove some of the old white coating from the original roof since much of it was crazed and loose anyway. Since I was recoating it that didn't matter. The new coating covered everything with the recommended 2 coats. The instructions for Dicor roof coating suggest to use a pressure washer with 2000 psi to remove the cleaner/activator that is used prior to recoating. My PW was only 1500 psi, but by adjusting the distance from the nozzle to roof it worked well.
If you have more technical or in depth questions Dicor will be able to handle them and they get back to you the same day with a response. Also their web site has videos that describe how to do it. I am not promoting their products, just think they were pretty helpful with my decision to do this. https://dicorproducts.com/
As cmcdar stated, research the correct method to use. I have EPDM roofing, and there are products for different types of roofs. But since the question originally was about pressure washing an RV roof...