If the goal is car camping, then maybe changing the paradigm would help.I have a heated jacket liner and gloves for my motorcycle.I've ridden comfortably for hours down to 15 degrees. On one maxi-scooter (Yamaha Majesty 400) I wasn't certain the battery would handle the load, so I got a marine battery to put under the seat.It would last for over 12 hours at the highest heat setting.

There are electric car blankets on Amazon for about $24. There are also small electric heaters. These would most likely last a night if powered by a marine battery.

I'd be afraid of any flame in the tank giving off carbon monoxide which can cause severe headaches or death.