Is the trucks gauge a voltmeter or and amp meter. A volt meter gauge would show 14-18, a amp meter gauge can show near zero or dead center. It will fluctuate when loads are applied and remove from the battery. You should see w jump for instance when headlights turn on. If this occurring then the system should be OK only when it stay 0 or stays to the left or right something is wrong in the battery + pos current line to/from the truck.
With the fact your only seeing 12.8 on battery with truck running leads me to think your regulator to the battery + has a problem in the line. But the confusing factor is the 14-15 volts on battery terminal to frame ground looks more to the ground lead. That would indicate issue in the battery terminal connection itself.
But one other factor to consider the battery does crank over the engine so the ground and battery to starter are good. So it has to be on the smaller battery to amp meter or regulator.
