Yes you can estimate that the furnace will draw about 7 amps per hour it is on.
The refrigerator, CO and propane detectors also will consume about 35 amp hours per day.
If you had a 120 - 140 watt solar panel, it would make up for about 25 AH daily in the winter shorter days, but more when it is cold out, the panels are tilted to the south, and sun is reflecting off the snow onto the panels!
You might also consider a Olympic catalytic safety heater. I have a 6000 BTU model and it keeps my 30' Bounder toasty warm at 30F outside, and with a little furnace use, I can keep it 70 inside and 19 outside.
The olympic heater does not use any electrical power, and has no fan or exhaust system. You would keep a window and roof vent open, just like when running your stove. . .
GOod luck,
Fred.