If you decide to use the pressure washer on the roof, which I don't believe is necessary, be sure to be very careful when the stream is directed at an opening (A/C, vents, whatever) as these seals were not designed to keep water out if that water is under pressure. I used one (with even less pressure than your's) on a motor home a few years ago and ended up with a wet ceiling inside.
All you should have to do is scrub it lightly with Spic n Span and then rinse it well. Use plenty of water to rinse so that the sides don't streak. Of course, if the sides streak, you'll have another job for the new pressure washer.