Typical furnace blowers draw 4 ish amps, and thus it will depend on your cycle time. Constant run for 8 hours overnight will be about 32 amps, plus whatever else you use. two nights 64 amps.
Keeping your battery discharge down to 50% of capacity, you would need at least 128 amps of battery for just the furnace if it is running all the time.
If you have 2 good quality deep cycles in good shape and are careful you may be able to pull it off. A more precise answer requires more precise data, as in what is your cycle time at 30deg, how much will you need the furnace during the day, what other loads might you be using etc.
2007 Chevy 2500HD Classic LBZ Duramax/Allison (Stock)
2005 Bigfoot 1500C8.2 AWESOME