Hi,
The control boards for the fridge require 12 volt power. In a unit that old the "converter" needs to have a battery connected to it.
The furnace fan runs on 12 volts.
Not many folks have appliances from 1975. They may also be millivolt systems.
I'd be having the furnace heat exchanger checked out. It may be rusted through, in which case I'd look at replacing it with a Platinum Cat.
Regards, Don
My ride is a 28 foot Class C, 256 watts solar, 556 amp-hours of Telcom jars, 3000 watt Magnum hybrid inverter, Sola Basic Autoformer, Microair Easy Start.