What is it you want to know? There is not likely a specific service manual for your bus, but the cummins is a very common engine, as is the Allison transmission. You should be able to find good manuals for them, though some things will be different on your bus, like locations of resevoirs, routing of hoses, etc.
Besides, buses are always a bit of mystery, wiring, plumbing, etc. It's supposed to be that way, I think.