On the road we once had our van washed, waxed and detailed at a KOA in South Padre Island by an independent service with a van that comes around to campgrounds. They did a fantastic job but it was not cheap.
A Mercedes Benz dealer washed our van when we brought it in for service once and didn't charge us. Once, I suppose because the van was dirty and they probably figured out it was better for servicing to clean it.
Advanced RV sometimes washes our van for service. Again, it probably depends on how dirty it is. They have a fetish for maintaining a clean facility.
Other than that our condo garage complex has a heated community wash bay with power wash wands of soap and rinse, a tall movable step ladder, pole brushes, etc. It is big enough for Class As, 5th wheel trailers and sailboats which are stored by others at the complex. Obviously it dwarfs a Class B.
Cost to use it is $1 for 5 minutes with the soap and rinse power wash machine. With two wands on both sides we can usually accomplish a wash and rinse for $2. Available all year round. We most times wash when when come home and put the van away in our heated garage. If we avoid gravel roads it would be rare that we bother to wash on the road. We have gone back to a metallic silver van we had with our Great West Van which has hid the dirt best over white and dark grey.