12V - 120V Norcold Fridge.

Last weekend my electric fridge stopped working on 12 volts.It still runs on 120 volt. I called Thetford but they have no parts for this unit anymore. It is 29 years old. Could the problem be the relay which switches the power supply from 12 volts to 120 volts in the power supply unit be the problem. There is 12 volt power coming in.
I know the fridge is old and I should buy a new one but $1500.00 US is a little rich for my blood especially when paying in Canadian dollars.