I agree, though the furnace runs on propane, the blower consumes a lot of energy. If the batteries go dead during the night, so does the fridge. At that point it can take forever to bring the batteries back to a reliable state.
If waking up in the middle of the night with a cold rig and dead batteries, it is best to either run the main chassis engine or the generator, both for heat and recharging. But running the generator in the middle of the night is not the neighborly thing to do, so I favor idling the main chassis engine. Just make sure you don't asphyxiate yourselves by leaving a window or roof vent open near either exhaust pipe. Make sure your detectors work properly.
Regarding maintaining inside temperatures, some rigs have a huge interior volume, one or more leaky slide outs, are poorly insulated and have leaky single pane glass, so their furnace is going to work hard, unable to keep up with the demand.
Our rig interior volume is little with a low ceiling, our rig is well insulated and we have thermal pane glass. We have no slide outs either. If not for the front cab, we'd be sleeping in a thermos bottle. Our furnace is very effective maintaining inside temperature when freezing outside. It cycles on and off with good rests in between. It helps a lot that the thermostat is in the bedroom and we keep the pleated privacy curtain closed to help block the cold from the front cab area.
Our tiny bedroom here.
With the curtain closed.
