Nope, I can't see any advantage to it. hard to get it up there and you don't need that much pressure to wash the roof. It could force a leak. I have a pressure washer that I use on my truck and the sidewalls and endcaps of the Coach, but not the roof.
I use a long handled scrub brush up there and a regular garden hose end sprayer. I use Borax and a tad of bleach. After its dry I apply a coat of "303" UV protector and buff it. my roofs look good forever.
Mike