[Tiara Open] 2006 36' Cummins QSB. Batteries test good with load tester,but dont hold charge

[firstlight]

Three house batteries test good with load tester AGM 31 series two years old,  Alternator new last season also test good. When at anchor for two or three hours with just electronics and one refrig on the batteries, as indicated on helm and cabin gauge drop to 10 volts.  However if the batteries are then checked they read 12.5 volts and show 97 percent capacity.
Checked all connection clean and tight.
Don't think the vessel has a battery isolator and I cant find the voltage regulator and not sure if they could be the culprit.