If your furnace is shutting off because if low battery, you are running the risk of shortening the battery life.
If you're running the furnace a lot, and not getting a lot of sun, try running your generator an hour or two in the morning, and an hour or two in the afternoon if you're not topping out with your solar. Looking at the weather for Bend the last few days, I doubt you're getting a lot of solar power...
Our furnace fan slows a little as the battery capacity drops, but it doesn't cut out til 10.6 or so, well below 50 per cent. The fridge won't work, either. They're built in safety features, but, that's still really low, and a number of days on end , depleting your power, will eventually shorten your battery life..
Cold weather boondocking is certainly challenging. Stay warm. We usually turn the furnace way down at night, and pile on a good warm comforter. I also like flannel sheets for winter camping. We try to keep the trailer somewhere in the 50s at night. Easy enough to reach over and crank up the furnace in the morning, before getting completely out of the nice warm bed.