Climbing in elevation means colder temperatures however there should be enough heat and circulation in the cooling system to prevent freezing.
Semis often use zip up curtains to partially block the radiator and I have been given several reasons for doing so including preventing the radiator from slushing up in extremely low temperatures at high speed.
FYI everybody, the maintenance schedule calls for a coolant drain and fill at 10 years or 150k miles. A 2015 should not need a coolant change. I would at least check the concentration of the coolant and the clarity with an inexpensive coolant tester. If it is not cloudy and good for the lowest temps in your area, you do not need to change it and you certainly do NOT need to flush it.
Flushing could ultimately shorten the life of the cooling system. It is best to drain and refill, but the subsequent changes should be done at 5 years or 100k because you will leave a little old coolant behind each time. The old coolant is less damaging than the minerals you may get into the system from flushing tap water through the system and FAR less damaging than the flush chemicals sold on the market.
Let us know when you replace the radiator cap and it solves the problem for you.
Some of the members here believe that 10 year coolant needs to be replaced every 5 years. 10 year coolant has been around for 15 years, I have not seen any negative affects from it.
Sorry it took me so long but I did replace the radiator cap and it made no difference.
I did recreate the problem and this time I noticed that when I got into the climb the transmission shifted and raised the engine RPM to about 3800. That’s when the temp gauge started moving up. So it seems the cooling system is not capable of keeping up at that RPM.
Could you please explain that comment?