The west coast is death on washing right now because of the drought, but typically we find some that allow, some will charge the $5-$10 and some just say no. I've never had one that wouldn't allow me to wash the front of the coach to get the windshield clean. Occasionally that creeps around the side.
In may, we camped at Watchman campground inside Zion Park. They only offer electric. The only water is at the dump station, but there is a small creek that zigzags through the campground. I was able to quietly wash the coach, a gallon at a time. :R
Since we live out west and do most of our camping out west, often your coach doesn't get that dirty, just dusty. We did some wet weather travelling in May and now I see what a lot of the East coast travelers deal with.
I didn't want to spend a lot of money on a pressure washer or carry a large one around since it wasn't go to be used that often. I went to Harbor Freight, bought their cheap 1600 psi electric washer and stripped all the plastic away. When I was done, the unit was the size of a football. I mounted it under the front hood, bought a 50' hose and made a quick connect for the water. The little unit came with a 25' power cord that I ran down the frame to an outlet in one of my bays. The cord has a built in breaker. The 1600 psi is powerful enough to clean without tearing up the exterior (adjustable nozzle). $75.00 for the power washer and $70.00 for a quality 50' hose.
Here is the unit:

Here is the unit stripped and mounted to the frame:

I made a holder for the hose and it all stays tucked under the hood:
