Backyard Charging Tests failed

[kensousa]

I've been driving this truck for a year with no issues.
Suddenly while driving to work, my headlights are getting dimmer by the mile.
Get to work and battery is dead.
Since I have no tools at the office, I throw a new battery in her and drive it home.

With the truck not running, fluke meter shows 12.6 volts across the battery so I think that's normal.
With the truck running, I'm only getting 12.5 volts instead of what I think is supposed to be closer to 14 volts.

So now I assume the alternator is toast. However, I was taught that if you put a wrench behind the boss at the center of the rear of the alternator (while running) and the boss is magnetized, that the alternator is working.

I get A LOT of magnetism with this test, yet I cannot get over 12.5 volts at the battery while running.

Am I off base with these tests and/or is there another test that I can do to help identify the culprit?

Picture shows the back of my alternator to help identify.

thanks