We spent the winter of 2018 at 7000' in Southern Colorado. It wasn't our Montana, but a similar SOB 5th wheel. We found the 2" foam board to work great. No extra heat under the trailer, and never a problem with frozen pipes or dump valves.
The whole wonderful experience (generally) is here in our blog:
https://smallhomebigyard.wordpress.c...019/?order=asc
Feel free to reach out with specific questions or issues.