We're full timers (going on 4 years) in a 4 season Montana. We've had our rig down in the low teens many times with no issues whatsoever.
Just off the top of my head:
Heating duct to the underbelly (under the basement only)
All plumbing is routed inside the insulated or heated areas
All wastewater blade valves are internal with remote cables (if it has external valves it's definitely not
a 4 season camper)
Disconnect fresh water hose and run off the pump.
Set the furnace to 40°F at night to keep heat into the heated underbelly under the basement and to minimize overnight propane use.
If one is boon-docking you'll need Lithium or generator as the furnace blower is hard on batteries. We have six Battle Born's so it's not an issue for us.
We always try to face the side with the big windows south to allow heating by the sun. The sun will do most of the work warming the place up.
Some good info on cold wether RV camping here: