Has anyone else run into this yet?
Its on my 97 1500 4x4. The truck does have a hitch with brake controller wired in. I noticed it the other night, at idle with the headlights on when you push the brake pedal the lights dim and the voltage drops from above 14v to below 14.
I checked voltage with a multimeter at the battery at idle and with the lights on it idles at 14.2-14.3v when applying brakes it drops to about 13.4v, add in heater blower motor and it drops even more.
The alternator was replaced within the last two years but it was before I got the truck so not sure what brand, new, reman, etc.

 

Have the alternator load tested. Most parts stores should be able to do that for you, on the truck.