Refrigerator at Sub-Freezing Temps

I just went to Dometic's website and found a similar model's operating manual.  I don't see any mention of a minimum or maximum operating temperature.

http://www.dometic.com/4e0e2322-4af5-4b4c-b50f-daa71478e943.fodoc

I also looked at Wikipedia's article on the ammonia absorption process and didn't see any mention of operating temperature.  However, it did mention that the boiling point of the refrigerant is around 0 F (-18 C).  It is a mixture of ammonia, water and hydrogen in a pressurized system.  I would imagine that the process will work as long as the refrigerant is above its freezing point.  The article didn't mention what that is, but ammonia by itself freezes at -77 C and boils at -33 C.  So I'm sure the freezing point of the mixture has a similarly low temperature.  Let's say it is -50 C, or -58 F.  There are few places in North America that get that cold and so the process should work anywhere you go, any season.  Hopefully a chemist can chime in here to confirm or blow this completely out of the water!

One other thing to consider is the heat sources (flame or electric resistance elements) don't output that much heat.  On the fridge I mentioned above I think the AC heater is rated for 175 watts.  Eventually the atmospheric temperature will be cold enough to rob the system of enough heat to operate the absorption process unless more insulation is added.