i read the section in my manuel that said don't pressure wash and didn't for years. (really didn't know why not).
one day i was in a hurry and put a relative wide nozzle on the pressure washer and washed. i have a few nicks on my decals now. the trailer was 6 years old at the time (i had purchaced it new). sense mine is exposed to the sun constaintly, i believe that and its age were contributing factors. i was using the 3 rd tip down and stayed at least a foot away. this normally doesn't produce enough pressure at point of contact to do a good job of removing dirt. the number 1 nozzle and 4 to 6 inches from the surface, i use to strip the paint off my house (2600# nozzle pressure), the wider the angle, the less the pressure at point of contact.
because i haven't had a good experience with the pressure washer, i don't recomend it. i'll stick with the brush and water. i really have better results with the brush anyway.