Best "mop" or utensil for cleaning coach sides?

I use a "Van Wash" brush, this is a big brush, may or may not have a flow through feature, it has a telescoping handle (Which you can replace with a push broom handle if you wish) works great.

Another item that might work (Fuller Brush sells this) is a window washer's brush. The fuller brush one fits on their "Multi-Handle" (mop. this thing, some other stuff)