+1 on Navmania's advice. Heavy cable, MUST be fused near the battery, forget the cig lighter plug it'll fall out on the third pothole and you will hit 10,839,456,382 more just on the stretch between Cessnock and Kurri Kurri, let alone any decent trip. By the time you arrive your butter will be oil and your milk will be yoghurt.
Heavy cables reduce the voltage drop that ALL wires experience, so the bigger the better. Protecting them is paramount - INSIDE the chassis rail if you're game, or cable-tied to the top if you can manage. Even on the road, you can snag a stick at a bad angle, flicking it up with enough force to tear through the wire, connecting positive to chassis. Protecting the cable from that is important, and protecting the car from fires with the fuse is even more important.
You might consider a voltage sensitive relay (VSR) so that if the battery voltage drops too low, power to the fridge will be cut out. I have a Waeco in the caravan and before I beefed up the cable size, there was enough voltage drop to make the fridge think the battery was nearly dead whenever the compressor started. It would switch off, and 30 seconds later try again. It did this so much at night it would kill the battery before morning with its attempts to start the compressor (heavy electrical load starting the motor, despite it being a "soft start").
