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HI my name is Robert...
Go to the round flue canister and look for a small disc with a tiny button on it, that would be a high limit cutout and would cut power to the fridge, you might need to remove a cover to get at it, it would have two smaller wires going to it. Many Dometics have that safety. What I also might do is get a battery and hook 12 volt DC direct, the reason for doing that is sometimes a power converter can put out unfiltered voltage and the circuit board does not like it. IF the voltage is good and the high limit is not tripped or you do not have one it sounds like the upper display is at fault.
Hope this helps some, let me know if you have more questions or concerns.... Robert