There is a ton of options to keep warm in the winter. There is also a lot of fact and fiction confusion along with opinions that make up most of the discussions like this.
1. Fact. Factory installed furnaces do use a lot of energy off your 12v system.
2. Fact. If you have one 12v group 24/27 battery with no way to replenish it, you'll be cold pretty quick running your furnace.
3. Fact. A generator properly sized can keep that from happening.
4. Fact. I hate generators.
5. Opinion strongly supported by fact....I use my furnace to keep warm in mid teens to mid twenty's with nothing but solar to recharge my 4-6v golf cart batteries. So it can be done with solar.
6. Opinion. Either a Mr. Heater Buddy, Big Buddy or a Catalytic heater is a safe way to keep your coach warm during the day or at night. There are concerns with ventilation and carbon monoxide poisoning that some education and proper use needs to be followed. Keep a vent open to prevent oxygen depletion if using a Buddy or something like that. If a catalytic or Buddy type heater is to be used there is not a concern with carbon monoxide if it is working correctly. But you still always need a vent open for oxygen. Proper combustion from a correct working gas heater of this type will emit carbon dioxide and water. Many people are spooked by this(as is my bride), so it is best to go with you're comfortable with.
7. Opinion. Generally speaking you don't have to have the temp all the way up to seventy when you turn in for the night. Heat the coach up nice and toasty, turn the 'stat down to 55-60 degrees to keep tanks and lines from freezing and pile some blankets on the bed. You'll preserve your battery power that way.
8. Opinion. I use a Buddy heater during cold days and in the evening while we're in the coach to keep in warm til bed time, then off goes the Buddy and we set the 'stat to 50-60 degrees til first A.M. when we rise. Then I turn on the Buddy again.
8. I keep a generator if everything goes south with my plan, but it hasn't yet and I hope in 20 years my generator is still brand new.