Mother Nature can wash my bus... every time it rains. We did wash it when we first got it. Took forever and it was exhausting work. There was so much mildew on it we couldn't touch it. So we hauled out the power washer and discovered some of the windows were not sealed. So we left the windows alone (and the moss) and cleaned it as best we could. Topaint, I wiped down the bus, a section at a time, with a white vinegar drenched towel (to cut the dirt and road grime) and deglosser.
I also don't wash my Jeep. It doesn't run well if it gets soap on it. Periodically David insists "R-Jeep" gets a soapy bath (in a car wash). But then the poor thing runs poorly for a week. Plain water is okay, but not soap.