Check with your local transit agency to see where they take their coaches. When I was driving for ours, I often pulled the duty in the evening to take the buses to be washed. Most smaller county agencies don't have a washing plant at their maintenance base, preferring to support the local businesses.
For those of you in my neck of the woods, Skagit Transit used a facility on the north side of Highway 20, just west of Burlington Boulevard, next to the Shell Station. I think their high bay is good for 18 feet, but check first. The SKAT coaches were about 11' high and seemed to have lots of clearance.
Frank and Eileen
No longer RVers or FR owners