I realize here in L.A. a cold morning is about 40 degrees (ha!!!).
But I notice that my voltage goes clear down to like 11 volts when I start it (and seems to stay at that voltage for a minute or 2) and slowly makes it's way back up to 14.2 taking at minimum 3 or 4 minutes. On a warm day, it get's back up to normal voltage quite quickly....I think less than 30 seconds.

 

There are two of those heater grids, and each will draw approx. 120 amps. On the post heat cycle (engine running) the PCM will cycle those grids, one at a time, until the truck reaches about 20 mph, or the temp comes up over a certain amount. What you are seeing on the charge gauge is normal considering that large an electrical draw.

 

sounds normal... that's what mine does.