Our winter camping experiences did not go quite as well as yours

Driving back from southern Utah to Wyoming just after New Year day, we pulled over at a Flying J truck stop to spend the night. We were dry camping having winterized in southern Utah before hitting the road. We awoke around 4 am due to being cold. The furnace was not working due to dead battery. Started the truck engine which warmed up the coach well.
After I got home I started wondering why the battery discharged so fast -- loads were only furnace fan, fridge compressor and a few lights. Did I mention that it was -20 degrees F. Took a look at the Lifeline battery specs -- at -20 F the batteries only give you 40% of normal capacity. To make matters worse the required charging voltage is above 15V when you are below 0 degrees -- since alternator puts out 13.5 to 15 volts (I have heard) I don't think the batteries were being charged as we drove down the road. Another concern is that the temperature at which you can freeze your battery goes up as percent charge level goes down -- at 25% charge level the batteries will freeze at 9 degrees F!
On the good side our 4X4 rig performed perfectly on many days driving on snow packed roads.