Batteries Not Included

I've noticed recently that each time we return to the boat, the house batteries are low. I went over to the boat with a voltmeter last Sunday, and took some measurements. The voltage was 11.5 on both batteries, and under load, the voltage started falling rapidly. Not good. So, it looks like I'll be buying some new batteries this soon... these are big boys, 4Ds, about the size of two car batteries joined together, it takes two guys to lift them into place. Who feels strong today?