I use a Big Buddy heater connected to the main gas line. I've only been able to test it down to 25F but it warmed up nicely there.
For batteries, I switched to 2 6V golf cart batteries in series. The big ones sold by Interstate carry some serious juice. They are aren't cheap but 2 of them will carry more juice than 3 big 12v's. The yellow Interstate golf cart batteries sold by Costco aren't the same ones. They only have about 1/4 the capacity of the big green ones.
Look at the reserve capacity (RC). It's the number of minutes the battery will produce a current of 25 amps before the voltage drops to 10.5V. The Costco version is about 125 min. The big ones are about 425 min.
RC is probably the most important number to look for on a deep cycle battery.
