Heater Turned on in Cab, Motor Home engine shut down.

I vote with K Charles.
Vehicles of this period had fusible links that were just short pieces of wire that were really too small, but had high temperature insulation. They fatigue, corrode or just plain break a lot. I just pull on the little piece of wire in front somewhere, and if it stretches and breaks, you have found the problem. The wire is available and any parts store.

Matt