I have a 68 litre Evakool fridge that I use in my Pajero. I put a second battery in myself and wired it via a heavy duty solenoid so it is parralleled to the main when the engine is running. I just used a Holden sized deep cycle (ED3) because I would at most leave the fridge running for one night only on the battery. If I am staying in one place for a great length of time I have a generator or solar cells.
I used a battery cradle that I got from Opposite Lock to mount the second battery behind the first on the passengers side.
The fridge will run all night on setting 2 (of 7) and still keep everything totally cold without draining the battery (I use it as a beer fridge at home and it will freeze beer if I crank it up above about 2.5!). I ran 8mm sq cable from the 2nd battery to the lighter socket down the back (10 amp fuse in line at the battery end) and I use this to plug the fridge into.

Hope this info helps.

Craig.