I have dug ditches in concrete with a pressure washer. They force of water is rather great.. Over time Ive seen water, just plain falling water (no pump behind it) eat rocks and such.
However pressure washers are "Hand to hand" combat tools at a distance of a 1 or two inches they can blast decals off, peal off bumper stickers, and remove paint.
At 2 feet they barley hit the wall of the RV.
Find the proper distance and they do a good job with very little water.
Be careful around windows and slide seals as they can blast past seals very easy
Or use a brush and bucket like I do.
A Van Wash brush, has an extention handle (Short it's a car wash brush, Long it's the same size as a window washer's brush) and works very well.