I can't say how your bus (or my bus for that matter
) is wired, but on some (smaller) vehicles a blown "alternator warning" light bulb on the dash will cause the alternator to NOT kick on.
If your "alternator warning" light does not come on when you turn the key from off to on (before cranking) that may be the problem.
Seems they commonly wired the vehicles with power from the ignition switch, through the bulb, to the alternator to signal it to generate.