I noticed on a couple cool mornings after starting my voltage meter would dip low and the headlights would go dim then it would return to normal. It would continue to do this several times until the truck warmed up. Is this normal?

 

Yes normal, grid heater is cycling which requires large amounts of voltage.

 

Normal for the Cummins.

See this for more info.
https://www.dieseltruckresource.com/...ad.php?t=57485

 

Yes, and when the weather's cold your idle may go up by itself as well.