Last I was camping, it was the end of Jan, outside temps were around 0F at night and in teens during the day. Fridge worked fine 2 days leading up to camping and 3 days while camping and 2 days when I got home but was too lazy to unload the fridge. I was on electric the entire time, I imagine the propane system would have worked without issue as well.
I'm sure there is some temperature point when the ammonia phase-change stops working but I don't know where that is. I've read on these forums of folks blocking the vents and putting light bulb heaters behind the fridge... not sure if it's out of necessity or just piece of mind.