Frozen drain line into septic from furnace
If the furnace was running in the home and the temperature in the home was above about 50 degrees F. I can not see how the drain line could freeze. Also, how did the drain line freeze all the way into the septic tank? Drain lines that are underground do not usually freeze. I believe that more information is needed as to the construction of your home, the location of this drain line, and the overall operating conditions  need to be explained farther. Is there a vent line between the house and septic tank that would allow cold air to enter the drain line at that point and freeze that drain line? Also, 45 degrees is probably a little to low a temperature to maintain a home when you are away. I would keep mine at 55 degrees.