Originally Posted by Steve480
...Should the heater (propane heat, but i guess battery sparked and dc fan to push air?) drain the house batteries overnight?...
In my experience two 6V batteries will not power the furnace fan for the night, unless it's not that cold in the first place. And if it's not that cold, why bother with the furnace anyway? Just throw an extra blanket on the bed.
Most people who boondock in cold weather use a catalytic heater for heat. These heaters run 100% on LP and do not use any electricity. Although they are not supposed to give off carbon monoxide, they will use up the oxygen in the RV so you need to keep a window cracked.