How are you painting the bus?
If you're spraying it, then I would prime the whole thing first.
You'll have to tape and mask most of it first anyways.
More importantly, if you prime the whole thing, then the whole thing will take the paint the same way. The way I've been taught to paint (especially when spraying) is that the first color coat over the primer is an intermediate coat - so you want the primer to begin to pick up the color of the paint, but you don't want it to be the full, solid paint color yet - that should come in on the second coat. I've had decent luck painting smaller things (a small tractor or two) this way, so that it keeps the individual coats from getting too thick and running/cracking/orange peeling.