Before you can determine how long the battery lasts, you need to know your load.
For example, the propane heater/furnace in our new 2514Gs is a high efficiency model: 2.7AH.
My camper, for example, came with a Group 24 DEKA battery rated at 75 Amp Hours. So,
So, if you had the furnace on at night but it only came on for 15 minutes per hour it would be something like this:
8x2.7 / 4 equals about 5.4AH per night. If it ran for 30 minutes per hour you'd have 8x2.7/2 or 10.8AH used per night.
That's just the furnace. You'll also be running your water pump a little. Plus the lights and any 12v devices you may have plugged in.
Due to something called Peukert's law, the more dray you put on a battery, the less power you'll get from it. Also, temperature greatly affects the chemistry inside a battery so if it's so cold you're running your battery a lot, then you probably won't get the full efficiency from it.
Also, you don't want to run a 12 Deep cycle battery down below 40% if you can help it.
I just ordered a 100W Renogy Solar Suitcase from Amazon. Should put out around 5AH and keep our setup topped off very well. We're heading to yellowstone next week.