A 30,000 btu Suburban furnace uses 8.5a. Figure 50% run time and 50% of batt. cap. gives about 11 hours on a 100ah batt. Your math may vary but they are power pigs.
We have 5 ah batts, 200w solar and run a res. fridge through a 2000w inv. (no fan heat), 13"tv, XM, a few led lights and in those same 3-4 days I see 12.0-12.2 v and recharge then.
Don and Mary