We carry a 2000 psi elec water pressure washer in our RV since we are full timers. I can fill the water tank, fire up the generator and power wash the RV/roof anywhere I desire and can do it without getting into trouble.
I like to power wash the roof with just water, no chemicals. My thought is to use full pressure and if the roof is going to leak or get damaged, I would rather find out then vs later during a storm!