You could add 2 more 6 volts as one larger house bank of about 400 amp hours and install an Engel 35 refer/freezer. The price is about 650.00. Less than an installed system. For that price you could upgrade both your batteries and buy the unit. We installed one as a second (redundant/bacup) unit and we love it. Use it to put all the drinks in. Uses very little power, stores well, works on 12 and 110 volts and you can take it out of the boat and use it anywhere you want. Crank it all the way up and it's a great freezer.

We had met a couple of folks in the Bahamas on a Catalina 30 that had one stored under the dinette.

We bought ours from John at Hotwire (the Kiss guy) Can't recommend the Engel 35 high enough.