[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.

[jclark003]

if your batteries are all charged, showing 12.5v then the problem is most likely in the wiring between the battery and devices. i would keep the load on and start working backward from where it is showing 10v and check voltage at each connection point. That your refrigerator and electronics both show low volts would seem to support a problem at or before breaker panel. So that is where I would start. If you show +12.5 then the problem is upstream in the wiring. If you show low voltage its downstream (disconnect switch, battery cables, etc)

[Sans Peur]

I have had wiring that looked good create a good amount of resistance. Test the batteries at the terminals then at the main switches. The wires are in a very moist environment and can wick that moisture up a good ways. As current is drawn through it will start heating up and cause a voltage drop from the resistance. As JClarke003 said Meter Everything in the stream.