Not sure if i understand your question, but I'll give it a shot. The lights, vent fans, refrigerator thermostat, and the furnace thermostat use 12 volt electrical power to operate.
A power converter is used to change the 110 volt electrical power to 12 volts to operate the aforementioned items. The 12 volt breaker panel is supplied by power by the converter.
If you wish to use only 12 volts, simply power the 12 volt breaker panel with charged batteries.