I have GMC Sierra 1500 SLT 5.3 FF...Replaced battery...Battery check always reveals 12.6 V....Alternator indicates 14.78 but then drops slowly to 12.66 consistently and holds 12.66 output. Before battery was changed Alternator charged at 14.2 V. Would appreciate comments on this.

Welcome to the forum.
I'd like to see what happens to the voltage when you put it under some load.
Turn on high beams, blower fans on high, wipers on, radio up etc. Does it still hold 12.6v then? Or what is the volts reading then?

Also how are you reading the volts from the dash gauge? or OBD2 ? Or from a VOM hooked up onto battery terminals?

Reading from multimeter direct to battery post. Under full load reading was 13.94. Drove several miles under full load and re-checked. Reading was same 13.94V.

It is a 2016 with 36000 miles.

Something'* does not seem right, Interesting, when loaded the output is a bit better,
Unless the PCM is real sensitive & limiting the ALT'* output, keeping the battery from overcharging, testing it would be next step.
You have a 5-year/60,000-mile warranty I believe.
Take it into GM and have them test it & see what they say, report back.

Sounds normal to me.

I agree, seems okay to me.

Thank all of you for your help. This is what I've learned. The new alternator'* produce only what is needed. With my old battery it needed 14.2 to keep things going. With the new battery it only needs 12.6 unless under full load and then it needs 13.9. When I had the alternator checked at the parts house it was 12.66 and they said I needed a new alternator but they apparently don't understand the new "Demand"alternators. That'* when I came to you guys. I bet they sell a lot of alternators that are not needed.