Furnace fan and battery drain

I used to run 2 group 27 12v batteries but I found as the outside temps went down the colder the batteries became as they need to be vented to outside. As the temps drop the amount of amps hrs available from the batteries would also drop and at the same time the more the furnace needed to run in order to keep up to the colder temps. The other problem is a cold battery charges very slowly. I solved most of my issues by going to two group 31 AGM batteries and moved them into the heated side of the camper and insulating the outside battery compartment door. This has doubled the available amps, I can easily go two nights of 10f temps outside and 70f inside (I don't like sleeping in cold) Two hours of charging by generator or truck alternater each day and I can boondock till the propane runs out. I also have 4 guage cables from truck batteries and camper batteries.