Concur with Jim above - on my '80 53MY there is one compressor/two air handlers for the salon, same for galley/bow. Then the master and guest SRooms each have a compressor and single air handler for a total in the boat of 4 compressors and 6 air handlers. There probably are differences depending on the year.
My galley/bow handlers have always been noticeably different in cooling capability, with the galley AH putting out much cooler air than the bow handler. However, since they both cool adequately, I have never really looked into it. In your case this change in ability occurred rather suddenly, which clearly indicates something has changed.
Am I correct that the forward air handler is blowing the same VOLUME of air that it always has? If so, that would tend to eliminate a clogged filter (if it has one - probably not) or clogged evaporator. However, it would be good to check that just to be sure. The access to our forward salon AH is from the galley cabinet on the aft port side. There is an access panel in the back of the cabinet.
It may be that if a dual handler system is slightly low on refrigerant, it affects one air handler more than the other - the furthest one, I would think; but that's just a wag on my part. It also seems logical that if the AHs are connected in series (I don't know if they are or not), then it makes sense that one would always blow colder than the other and that if there was a recent loss of some refrigerant, it would affect the furthest one the most. BUT this is just speculation on my part.
I would check the evaporator for clogged filter/fins and if all looks OK, as much as it pains me, I would suggest you consult someone who regularly does work on such systems (dual air handler/single compressor).
