My only worry with fridge connected to starting AGM batteries is that that they are starting batteries and not deep cycle batteries. They are designed to be kept mostly charged and not cycled. How much battery life are you taking out by connecting the house to them and then deep cycling them (say 50%). Admittedly, a small fridge is probably only about 30-40 Amp-Hours per day. But then, the dual starting AGM are only about 100 Amp-Hour total (correct me if I'm wrong as I did not look this up again).