Incidentally... if you've ever noticed a big CLICK when you turn the ignition key to the run position, that's the sound of a constant duty solenoid buried in the wiring box to the left of the driver seat. It looks like a mini soda can with several heavy and several smaller wires attached to it. A key switch beefy enough to carry all the power used by lights, body heaters, etc on the bus would be impractical so instead they have the key switch operate a solenoid located in the wiring bay.
By the way, "constant duty" is important. A solenoid that isn't marked for constant duty will likely break (its electromagnet coil will burn out) if left powered for long periods. The constant duty types are often built so that after pulling in they automatically reduce the power through the electromagnet to help it run cooler.