[jsb5717]

Welcome aboard! You've received good input. You might get some satisfaction out of your fireplace while plugged in to shore power, but not enough to heat the whole rig when temps are that low. The gas furnace is likely your only viable option since the heat pump won't help you below the typical 40 deg threshold.

I also agree that it sounds like your batteries are failing. You could remove them and have them load tested just to be sure.