To quote page 9 of the Norcold 12xx owner's manual:
When you operate the refrigerator on propane gas at altitudes
higher than 5500 feet above sea level:
- You may experience reduced cooling performance of the
refrigerator.
- You may experience burner outages.
To avoid these possible problems, Norcold recommends that
you operate the refrigerator on AC when at altitudes higher than
5500 feet above sea level.
Since I think the main problem might be reduced oxygen at that altitude, have you tried adjusting the burner assembly to allow more air? Just remember to adjust it back at lower altitudes.