2006 GMC Sierra Dead Batteries

I have a 2006 Sierra 2500HD 4X4 with the LBZ Duramax/Allison setup. It has about 38,000 miles. Twice now the truck would not start in the morning. I recharged the batteries(truck has 2 batteries) and the next day the Duramax would not turn over. In both cases I checked battery voltage and it was low. The only item that seems odd is the Voltmeter gauge. It will show charging above 14 volts for a few minutes after you initially start the engine, and then it will drop below 14 volts and vary up and down in the 11-14 volt range while driving the truck. Turning accessories on or off does not seem to effect the Voltmeter. Is this gauge indication normal for a GMC? Do I have a bad battery/batteries (they are only 3 years old and original to truck) or is the voltage regulator shot. Any help would be greatly appreciated.