lows below freezing, bad idea to camp?

I need to run the furnace since we have two little ones with us (3yrs and 8 months). I have dual 6v batteries and a generator so I should be fine. Since I went to this set up I have not woken up once to a dead battery like previously. But I also never camped this cold so Im sure the furnace will be getting a work out.

Thanks of the latern tip. Never thought of that. Seems dangerous but I guess if everything was vented it would be fine. I dont think I will try it though. Too much for me to lose if I did it wrong.