Question:
Have you confirmed that the furnace and interior lights will run on 12 volts? If so, what was the original source of 12 v power.
One or two extra batteries in the truck bed and a switchable connection to the truck charging system will get you on your way. A small battery charger will keep you going when you have access to shore power. A small power inverter will service small electronic chargers and power supplies.
Solar panels or small generator will extend boondocking time.
Was the pantry originally a refrigerator or just an ice box?
Hope this helps,