Generally you just push the electric switch for 120v usage. Pushing the gas switch works if the fridge has DSI ignition (it lights itself), but doesn't if the gas burner requires manual lighting. I can't answer that without knowing the fridge model cause both type were around in 1983 (newer ones are all DSI).
In LP gas mode you only need DC electric, probably 1.0-1.5 amps. In 120v electric mode you still need the DC, but you also need 120v electric for the heater, probably about 350-400 watts. Solar can't provide that directly, but you could perhaps use an inverter from a solar-charged battery. It's possible your fridge also has a 12v electric mode, but we need the model number to determine that too.
Look inside the fridge door for a sticker; also open the outside access panel and see if there is model info back there.