The refrigerator in my trailer is a Norcold.
I called Norcold and discussed this with them.
They said that below 32 degrees (F), the refrigerator would not operate correctly, and that at temps of ~ zero (F), damage “could” occur. (I have used my trailer in below zero temperatures and, so far, the refrigerator still works)
However, they now make (and install on all new Norcold refrigerators), a "Cold Weather Kit" (Norcold part number 634913) that will allow the refrigerator to safely operate down to zero degrees. I am still unclear as to exactly what it does; however I will probably get it just for piece of mind when operating in cold temps. The price is about $80.00. If I do get it, I will post my feedback here.
Steve