My furnace blower takes between 7 & 8 amps How long your batteries last will depend on how cold it is, how much area are you trying to heat, and the thernostat setting (those drive how much time your furnace is operating), and the size and condition of your battery.
As a data point - my first Airstream (21') started with one Group 24 battery. On a typical day in late autumn where the overnight temps would drop to about 30, and the thermostat set at 65, this one battery would not go through one night. I added a second Group 24 battery, and then it could go through one night, run the lights & water pump the following day (didn't run the furnace during the day since it warmed up enough outside), but could not make it through a second night with the furnace running.
I solved this problem by adding a catalytic heater that doesn't need battery to operate (but made me be concerned about carbon monoxide, etc), and then eventually a Honda protable generator - so now I just recharge each day (when camping in cold weather).
I also installed a meter that tells me how many amps I've used - doesn't give me more capacity, but gives me a better idea of battery condition so I can relax and enjoy the camping!