I don't know about an undervoltage relay, but I think something could be made pretty easily.
If you run the frige on the load side of a relay, with the control side run in series with a zener diode, I think you can get that to work.
A zener diode blocks current flow until the voltage goes over whatever the rating on the diode is. If you can find a zener rated at 11.5 volts, and that will flow enough current to operate the relay, you should be in good shape. I don't know if you can get anything that will go off at 11.85 and back on at 13 without finding someone to design a circuit specifically for that purpose.
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John USYacht 27 "Cora Lee"