There have been many discussions re Norcold refers not cooling. A "Search" on keywords like "Norcold, cooling,etc" should find many of them.
A couple of questions. Is this problem occurring on both electric and propane? Is the thermistor attached at the top of the fins? What is the ambient outside temperatures? Are the exhaust fans running?
This is too new of a unit to expect a cooling system failure, Not that it can't happen, but other causes are more likely.
There is a " Norcold 2118x Service Manual" in the Files section located here:
https://www.montanaowners.com/forums...?do=file&id=72 It should help give you an idea of what you are looking at. I seem to remember a diagnostics manual somewhere but I haven't found it - yet.