Interestingly, 12 and 24 volt, compressor based refrigerator / freezer combos have become quite efficient and not all that much more than some of the higher end coolers. The main power draw is cooling the food down, so if your items are already very cold, maintaining the low temperature is within reason.
The ones that are based on thermoelectric are unfortunately not that great, but the compressor based ones are pretty decent. They are very common in the RV / conversion van / commercial truck market.