Insulation values get higher as price of the rig gets higher. Montana can get pretty cold. You might want to see what's available in the Bighorn line.
You should be able to stay warm with furnace and supplemental heat, but energy cost can be high. Propane pricing went through the roof this past winter. We spent 7 weeks in Breckenridge, CO in our Landmark and it was what I would consider a mild winter for Breck. Propane went up to $3.30/gallon, and that was on a resort long-term contract price. Retail was over $5.00/gallon. We use about 10 gallons/week. Electric was substantial as well.
You'll want to have the local propane company deliver an external propane tank - at least 120 Gallons if not larger. In Montana, you'll probably want to make some window panels for insulation, especially at night.
If you put a space heater in the basement, be aware that the heat will rise behind the walls and may confuse your living area thermostat. If you have to turn the thermostat up to 95 to get the furnace started, that's the reason.
Also, take special care when draining tanks to make sure there are no leaks. I have a clear twist-on adapter and external gate valve. After dumping the tanks, I close the external valve and check back in an hour or two to make sure there's no slow leak. If you have a slow leak in very cold weather, you can build up an ice dam that backs up all the way to the tanks.
Also suggest you think about backup procedures for your rig in the event you have a furnace problem or electrical failure. For example, if you have an electrical failure in the trailer, you'll need a high capacity battery charger to keep your 12V systems running, including the furnace. But if your tank heaters are 110V, you'll need to dump the tanks to keep them from freezing. If you have a furnace failure, will you have enough electric heat as backup? If there's a local electric power outage for a day, you'll want a generator to keep critical systems going until power is restored.
