I was our bus myself a few times a season, more if I drive through lots of rain or dirty roads.
A few things I found make it easier...
- Use softened water, especially for the rinse. I'd use RO water if I had enough and a way to pump it through a sprayer.
- Use a non-spotting agent in the final wash water.
- I use a small cordless blower to get rid of most of the water on the skin before I attack it with the towels. This makes the drying much less difficult.
- Of course, wait for a nice cloud-covered day so the sun doesn't beat me to the other side.
I wouldn't trust a car wash to clean the bus without damaging anything or getting water pushed into the bus interior or the bays.