2006 Silverado voltage fluctuation

[Collins803]

2006 duramax 4x4. My battery voltage gauge is bouncing around between just under 14 and just over. Batteries were recently replaced with pair of interstate’s. I mainly notice at night with lights on, all my lights inside and out will get brighter as voltage goes up and dim when it lowers. 
 

alternator shows to be charging normal with a multimeter, alternator is about 5 years old 

 

any help would be great on what I can check. I also have led interior lights and a few exterior of that effects anything.