The amp hours used by a 12V fridge per day depends on many factors, but many of the best 12V camping fridges use around 1 amp hour per hour or about 24 amp hours per day. A typical car battery can run a fridge like this for a day or so without too many problems, but longer than that, you should use a different power source like a generator or a portable power station with or without solar panels. A small solar panel with a portable power station can likely generate enough power to run your 12V fridge endlessly since they usually draw about 45 to 60 W at most.