We have stayed many times in our Diplomat in absolutely freezing weather in both Breckenridge and Park City (love the skiing in both places btw).
The build-in options by Monaco will not do the trick once it get just a little bit cold, and there are only two options that will work (imho).
1) Propane heating using the furnace (if your propane furnace will work at altitude which many won't), or
2) Space heaters (if you got the AMPS required).
I hope you find a way to stay warm