I have a 2007 new body style Sivderado HD 2500 6.6L diesel with dual batteries a Bosch alternator and only 40k miles. The problem I'm having is it takes over 3 minutes from a cold start for my voltmeter to move from the 12 volts mark. After nearly four minute the volts go up to 14+ and stays there while I'm driving or parked. If it sits for a while it is delayed again. I first noticed one night my headlights were a little dim and I notice the meter, after a few minute lights got brighter and the meter went up.

 

That may be normal operation. I had a truck just like yours and it and my new truck do the same thing. At times the meter is below mid point and appears to be discharging. Then for no apparent reason, it moves up to charge. I believe I saw it mentioned in the owner's manual that this is normal.

 

We had an 06 Duramax that did that for awhile and then finally both the batterys just quit holding a charge. I'd get them tested. Diesel's strain those battery's pretty hard