NEW ALTERNATOR?? WONT CHARGE BATTERY?? CHECK THE FUSABLE LINK TO THE POSITIVE TERMINAL. ITS NOT THE ALT OR THE VOLTAGE REGULATOR (WHICH IS INTERNAL)

Did you check the fusible link or not? You CAN hook a thick, Fused (!!!!) cable direct from the Alt. + terminal to the Battery no problem, just leave all the other stuff hooked up because of minor systems like the stereo etc are using them. This can bypass a faulty alternator fusible link or mouse chewed wires.....

Are you quite sure that the energizing plug is hooked up on the back? It provides power to the stator, creating a magnetic field, which makes an alternator work. If this plug is not hooked up IT WILL NOT GENERATE ANY POWER.

With the engine running (maybe off a jumper vehicle) you can test the function of an Alt. by holding (VERY CAREFULLY!!!!!!) a screw driver by the positive lug on the back of the Alt. A magnetic attraction should be felt!

DO NOT LEAN ON THE BODY OF THE TRUCK WHILE DOING THIS! UNLESS YOU ARE CURIOUS WHAT A CAR BATTERY FEELS LIKE SURGING THROUGH YOU. MAYBE WEAR SOME RUBBER GLOVES AND DEFINITELY DON'T STAND IN A PUDDLE

What is the Voltage measured at the Batt with engine running? How about with it off?
Now measure the volts with ground on battery and the positive lead on the positive of the Alternator.
Are these readings different? this could indicate a faulty connection, ground, burned up wire etc.....

Also the oldest trick in the book is to remove the positive battery cable while the truck is running, if it dies the alternator is not functioning as it should (although may not be broken, just hooked up wrong).