Dry camping with furnace on

You need two or more typical 12v hybrid batts, or a couple of 6v golf cart batts to keep your furnace running longer than 8 hours.

I bought a digital volt meter from China that I connected to the plus and minus on the 12 volt panel board. Now I check and when there is no load connected, I know how much battery is left. After a while you learn how long your batteries will last by watching the voltage drop.

I think I paid about $7. for the meter, delivered in about 5 weeks from Banggood.com. Get one that has a four digit display. It's good to know the difference between 12.34 and 12.26 and a 3 digit display would just show 12.3 in that situation.

you say you only have 1 battery, what size?
I had 2 27 series batteries, when we went to yellowstone, back in '08, it was 32°, I set the furnace at 65, (big mistake), dead by early morning. I now have 2 31 series batteries, & know better than setting the furnace that high, or for that long.
Also, watch your light usage, especially if you don't have LED lights.

Our furnace fan uses just under 7 amps. With that information, I can determine how long my batteries will last, and how long I can safely run the fan. While dry camping, I will check the status of my batteries every couple of days to see how they are doing. I shouldn't get any middle of the night surprises that way.

The blower uses a lot of battery. I would suggest using more blankets and off-set the temperature drop by using a portable catalytic heater that is RV/Tent safe. Good luck!!

Take the existing battery to an auto parts store that tests batteries for free (they can test it when while it's still installed). Then get either 1 or 2, depending on the test. Shop around - the auto parts store may or may not have the best prices.

How cold was it and how much did the furnace need to run? How much power does your furnace require?

An extremely rough rule of thumb is that a battery is good for a night's furnace usage, but there are so many variables it's hard to be at all precise about things without more information. It could very well be that your existing battery was on its last legs to begin with. It also could be that you need more battery power to reasonably get through a cold night's furnace usage. Rather obviously, turning the thermostat down during the night will reduce the furnace run time and economize on battery (and propane) usage.

If you have room (height, mainly) for them, a pair of 6V golf cart batteries (wired in series) will typically give you more usable battery power than a pair of 12V "deep cycle" marine/RV batteries in parallel. They're also more robust against the occasional deep discharge.