Electrical issues

I've been having this issue ever since I got the
truck, whenever I use headlights, tailights, turn
signals or heat/ac the voltage in my truck drops
significantly, if I use them all at once you can hear
a very noticeable slow in idle speed, this also
makes my interior lights dim, and after awhile of
driving it the voltage slowly starts to drop as if it
isn't charging, I've had the battery and alternator
tested and there both good (there also both less
than two years old), I've verified all grounds and
even replaced the wire that goes from the
alternator to the battery with a thicker gauge wire,
nothing has helped, my alternator has one wire that
goes straight to the battery and another small
green wire that connects to the alternator via
harness (only one wire on the harness) I don't
know what it's for or what it goes to, I hope
someone can tell me what's going on and help me
diagnose this