Mine is hard wired using an Engel Posi-Fit outlet. It locks the 12V plug into the outlet so no accidental unplugging of the fridge.
http://www.engel-usa.com/fridge-freezers/fridge-freezer-accessories/miscellaneous-accessories/engel-dc-posi-fit
Engel told me the fridge will average about 1 amp per hour...obviously that will vary with ambient temp, but it's a ballpark figure. Your battery (and mine) should have about 100 ah, of which 50 is usable (to avoid draining below 50% which can shorten battery life).
I have the Engel MR040. I like it. The plastic case is quite durable I think, and won't dent like the metal ones. I have a single battery system (Sears Platinum 31), and also am running a CPAP overnight, and have had no trouble whatsoever starting in the morning. I also carry one of those little...