A Lance 881 20,000-25,000 about 5-6 amps.
Cold weather will shorten the time you can use a battery. If you could install AGM's inside and keep them warm, you'll be ahead of the game.
Pull the slide in at night if you can, the furnace will cycle less.
The two stage furnace can be more.
http://www.rv.net/forums/index.cfm/fuseaction/thread/tid/651913.cfm I have two grp 29's dedicated to the furnace and water pump. I can make it thru one night and the next morning if I run my Honda 2k just before I go to bed.
I use a separate third battery for LED lighting as well as the inverter to operate the TV/DVD .
I charge all 3 batts at one time .