E,
The minimum recommended free space behind these units specified by the manufacturer cannot be had given the constraints of boat design. This lack of ample free air space around the condensing coils is the main reason these fridges work so hard. It's a fundamental engineering constraint that is difficult to completely overcome. Keeping those coils spic and span would be a good start though, and if the fridge has never been pulled out long enough to do so, it would be a good first step.