Either you have a bad battery or something is draining them. I would charge them up and then disconnect them by pulling the cables off of them for 24 hours and measure your voltage again. Check each battery for 6 volts as opposed to both together at 12. That will tell you if one of them has a problem like a shorted cell.
One other item that can pull your voltage down would be an inverter. Even with nothing plugged in, the inverter will use power.
WBCCI President 2016-2017