The gas flame heat output at high altitudes on appliances can be reduced, which is why Norcold has said that the refrigeration performance may be a problem. It is not an issue with the refrigeration system itself, but the heat input on propane. It would not affect the operation on electricity.
I don't have a Norcold refrigerator in either of my trailers, they both have Dometics. I don't recall any statement in the Dometic manuals about high altitude reduced operation but have not looked carefully for it. I camped this year for a week above 8700' and had no issues with my Dometic at any rate. My furnace and water heater also seemed ok, but both were a bit more noisy when the flame was on.