Hello Spiro,
The proper answer is that these 3 way powered fridges, are not designed to be run on 12V battery by its self. The normal mode is to have them powered by the car engine only when the alternator is producing power. They tend to use about 8 Amps of 12V dc current continuously, so they will quite rapidly discharge any car or caravan battery. (car battery Ah divided by 8A = theoretical time, but if often much less due to age of battery ad other losses) ,)
There are two problems you will encounter if you try to run them from just your car battery, The first is that after only about 1 or may be 2 hours, the battery may not have enough capacity left to start the car engine. the second is that if you did leave the unit taking current from the battery, as the battery car battery reaches the end of its charge, the plates inside the battery can be irrevocably damaged. Caravan (leisure) batteries are built differently and can withstand total discharges, but even they will be damaged or have their capacity impaired.
Check the manufactures specification for insulation to assess the effectiveness of the unit left un-powered, Freezer ice blocks can help keep things cool for things like ferry crossings.