'08 F250 Voltage Readings

[bjd76]

My fishing trips are usually about 250 mile round trip so I'm a little sensitive about things going wrong... Been wathching the voltage reading through the GPS for about 6 months: Port engine steady from 13.8 to 14.4v but the starboard engine is at 12.4 to 12.8v. Worried about an alterntor or voltage rectifier going out 120 miles out...
Replaced batteries, cleaned all terminals, and no change at all. Thinking I need to have the alternator and/or rectifer tested and replaced by dealer. Anything else that I can check before taking that (warranty) step? Thx in advance.

[99yam40]

what about taking a reading with a good hand held voltmeter at battery and at out put of regulator to make sure there is a problem with out put from motor