The MOST important thing is turn down the heat while you sleep. No higher than about 50. The heater fan will kill a battery's charge faster than anything.
When we boondocked in Yellowstone in September 2011 with 2 group 27 batteries, running the furnace at 65, we ran the batteries down by 3am. It was in the mid 20's outside. When we turned the furnace down to 50, we could get 2 nights out of them. Keeping it at 50 will keep pipes from freezing. When you get up in the AM, turn the heat up and jump back into bed for 10 or 15 minutes while it warms up.