There's a guy in my town with the professional equipment to inject expandable foam insulation in walls...
I believe I could drill a hole in between every ceiling rib (inside the bus) and let him inject the liquid foam into each roof cavity. It sure would be good insulation without much effort. It can be applied in walls with the fiberglas batts in place, I think it would work on a bus too.
I will call him and ask some questions...