Dead Battery

I have a 2007 Seirra with about 5,000 miles, 5.3L (The Fuel Mgt Eng) and the battery has gone completely dead twice. Once in February 08 and again in July 2008. Both times I took it to the GMC Dealer and was told there was nothing wrong with the charging system. The only item that seems odd is the Voltmeter. It will show charging after you initially start the engine and return to 14 volts after a few minutes. Later it will drop below 14 volts to what appears 12-13 volts and it may or may not return to 14 volts until after you stop and restart the engine. Turning accessories on or off does not seem to effect the Voltmeter. Is this normal for a GMC or is the alternator (with built-in voltage regulator) bad the they just don't want to replace it? Any help would be greatly appreciated. b road