Eli.
My voltmeter runs around 14 volts when I first start aout and then stays around 13 -13.5 after I have ran it for a while. With all lights on mine stays around 12.5 -13. Try taking a regular volt/ohm meter and reading right across the battery terminals to be sure that your voltage gauge is accurate. The voltmeter across the battery terminals should show what the alternator is putting into the battery. If the voltage still drops, maybe have a load test done on the battery. Also when your mechanic tested the alternator, did he load test it or just test to see what it was putting out with no load?
Ron
