[NYMarine1]

I have a 1997 3100 open with 454's. Here in the great lakes we troll at very low speeds (i.e. 1.8 to 2.8 mph). In order to do that I run one engine at a time. When I use the port engine my volts stay between 12 and 13 but when I use the starboard engine the volts slowly drop, the last time it was down to a little less then 11. When I raise the RPMS to over 1000 it begins to slowly rise. I am thinking it is the alternator, is there an easy way to check it? Could it be some sort of regulator that is not working properly. Any suggestions? 

I don't know about the 454's but the 8.1Ls on my 35 have voltage regulators that don't open/activate until around 1000 RPMs.  I think that is the way they are designed.  My port engine alternator charges the engine batts and my starboard engine alternator charges the house batts.  Until I hit 1K, the voltage does drop.  When im in the 5 mph zone I usually drop the starboard engine to 500 RPMs and bump up the port engine to 1K RPMs; probably don't need to though.