Both Norcold and Dometic make three way (12V/120V/propane) reefer/frezer products. The 3-way products do NOT chill or freeze on 12V, they simply maintain a 'pre-chilled' reefer to keep it cool. In 12V mode they generally require about 18 Amps per hour draw. They also require a <1 Amp feed to operate in propane mode - this supplies the computer with power.
I have a Dometic 120V/propane reefer/freezer in my Airstream 'hunting lodge' and it works fairly well on propane or 120V (Honda genset). I plan on getting a 3-way and have done some research, which is ongoing.
These are made primarily for RV applications where you do not want to be running propane while driving - generally considered bad ju-ju.