You are slightly colder than us currently, but I just looked outside and it is snowing (which was not in the forecast).
We are running 3 ceramic heaters (2 smalls box and 1 tower). We keep one of the small box type ceramic heaters in the basement. I only let the furnace run about once every hour during the day, at night I set it up so it runs a little more often.
Since you have RV antifreeze I would start putting a gallon in your black holding tank. Last year when we stayed in central Illinois during December my black tank valve froze shut. Once I got it thawed out and the tank dumped I started adding the antifreeze and never had another problem.