Norcold Refrigerator Help Needed - Won't cool at altitude

Sound to me like you have a problem with the burner. You should see a reduction in cooling. It shouldn't quit.

In addition, the 5500 feet is a guide line as to where the burner efficiency may start to fall off due to less oxygen.

If it's a new TT, head back to the dealer and ask them to take a look at it. Something isn't burning quite right.