I agree with Sailingdog here. Climate has an effect as well on efficiency, as well as your eating and "access" habits. If you are going in and out of a half-filled box for cold pop all day, the unit will run more than if you have it packed out with long-term supplies. On passage, you would open it as little as possible and would pack it in the order in which you expected to be eating, in order to avoid more amp draw to cool the tropical air you've let it four times a day.
The only constant is that you want the fridge full for best results, and you want enough of a surround in the cabinetry to insulate the hell out of it, and to put in a nice heavy lid. With a lock-down, of course, so you're not killed by a bag of frozen peas in a knock-down.