We use a standard deep cycle 12v marine battery from Wal-mart. We have been keeping it inside the cart (which is "well-ventilated"). For an enclosed space I would say AGMs although I am not comfortable with ANY battery inside. AGMs are pricey. To power a furnace overnight, you will need more than one battery (more like 4 well charged batts). I know an RV furnace will kill a decent sized battery bank over night (A decent sized battery bank is a lot more than one battery). That is why the boondocking RVers use LP CAT heaters. My suggestion for a one shot deal would be to run the genny all night. Is your tank big enough to run all night? OR stick a couple of kerosine heaters or LP heaters in there. For the low temps you get, I would say two heaters should hold you.