I find about 2 out 3 campgrounds allow washing, except in California and maybe Washington, due to the drought. When not available, I do what "FIRE UP" does and wash it where I can with a power washer.
Recently, I took a cheap Harbor Freight power washer and disassembled it. Here it is on Harbor Freight's website: 1650 PSI 1.25 GPM Electric Pressure Washer
I removed the cover and all of the superfluous stuff, leaving just the pump, which is about the size of a football. Here is a photo of the plastic cover.
This is the actual pump. I mounted it to the frame rail alongside my generator.
I ran a pvc pipe from the pump inlet to the edge of the body near the generator tailpipe. I put a standard 3/4 female hose fitting on the end so that I could quickly attach a water supply either from the campground or my own water pump.
The Harbor Freight power washer comes with a 25' power cord. I ran it down an upper frame rail into a bay where I had an outlet. I leave it secured near the outlet when not in use.
Lastly, I went to a local supply house and bought a 50' hose so I could walk around and wash my entire coach. I made a holder for the hose under the hood.
The entire project cost me under $200.00.