12V, air cooled fridge is very doable and has become the standard for cruising
boats (though there are some that argue for and love their
water cooled units).
Power consumption will be primarily based on how good your insulation and
seals are. The best, most efficient
compressor in the world is going to suck power like mad if you don't have a good box.
I have a 9-10 cu ft box with about 1.5 cu ft of that in
freezer space. I draw about 50 amp hours/day in summer with ambient temps mid 90s day, 60-70 night.
Winter I use half that. One 330 Watt
solar panel makes more than I need to run the fridge.
To get this I ripped out the old box and built a new with min 4" of Dow foam insulation. Also made a new lid with double
seals. Unless some PO of your
Morgan already did this you will need to redo the insulation. If the box is really big and you can live with less room the easiest method is to add insulation on the inside.