Where will you be in Montana? The whole state gets chilly in the winter, but some places are significantly colder than others.
I second the diesel heaters. I got a cheap Chinese 5K unit and it is pretty amazing. I'm nowhere near finished converting, so I don't have "real" experience with it, but it allowed me to comfortably work on the mostly-uninsulated bus all winter. After a few weeks with the stock thermostat, I bought the Afterburner control module (
Afterburner Website) and love it. Nothing better than rolling out of bed, turning on the bus heater from my phone, and making coffee in the house while the bus warms up.