Power Problems with 1996 Isuzu Trooper

I have a '96 Isuzu Trooper. When the accelerator is pressed, my battery meter drops to 12 volts when it is normally at 14 volts. At this point it takes a good 5 minutes in at the truck to 55mph. What could be causing this, and what do I need to do?

Any possibility the alternator belt is slipping, maybe a bench test of the alternator is called for. While the alternator is off, good time for belts.

But before you do anything verify all is well with the battery and connections.

I would work on this strange electrical system condition first (before the driveability)