A rig that old might have a circuit that comes from the IGN switch, through a diode, all the way back to the alternator supplying a Field Voltage. If there is a poor, corroded, or open in that circuit, you'll get your symptom.
Check your chassis drawings, and the original alternator drawings to see if that's the case.
Note that many RVs in that era switched over to an alternator that supplies it's own field voltage. There's a good reason why some manufacturers kept the old IGN switch to diode to alternator design and it makes sense, I've just forgotten what it was.
