2006 chevy silverado 4.8 4x4 125,000 miles

I noticed after driving 3 hours to buy a boat. Picked the boat up, got on the road and as the night fell so did my confidence in this truck.
I was driving with the ac on along with head lights. I noticed the dash lights and headlights diming like a vehicle with a large sterio system when the bass hits.. it dims down for just a sec then back to full illumination - at the same time the air conditioner falls off like it is being turned down then back to its origional position.
The battery gage is in the 14 range but will rise to 15 back down to 13 but nothing quickly. The tach is working as it should. No other issues with the truck.
I just didn't know what my first move was to be???

Thanks for the help !

 

1st...Welcome

2nd....First thing I would do is replace the serpentine belt. I may be just that it is slipping. I had a similar issue with mine.

 

I didn't think of that, Thanks !