Freshly charged batteries or charging batteries will show a surface or charging voltage.
You will need to seperate and disconnect them from any load, wait at least 4 hours and measure each ones voltage.
12.6 or 12.7 volts is a resting voltage of a 12 volt battery.
If the batteries spicific gravity, on parallel batteries, is different, a equalization charge may help.
I doubt that you have any way of forcing 15 volts into them for 4 or more hours, so my advice would be to charge each one ( by leaving one disconnected ) overnight, using the onboard charger.
Do that and see if they are about the same SG. If not, its probably replacement time, at least for the weakest one but probably both.