Rusty you may already know this some others may not.
Volt meter will only give you the state of the surface charge of the battery, it needs to be tested with a load, same with your alternator.
Think of it as trickle charge vs full charge still 12v read at the meter, but only mV of charging current being produced, the voltage regulator of the alt needs to be checked for ability to charge at load, GM alts are not the most robust of alts.
Bad batts kill or weaken an alt as for a while they will try to keep compromised batts alive and also try to keep the vehicle load supplied as well. Think of an air compressor tank, compressor fills the tank and air is used up the compressor maintains on and off cycling to keep up with the demand, bleed down the tank completely and your compressor works it's tail off to meet the demand until it is so over worked the motor trips a breaker on overload, or compressor mechanically it slap wears itself out.
To an extent your charging system is similar to that, batteries being the air tank to store up energy for times of high current demand, ergo glows/starter, dirty plates in the battery and they won't take the charge, dirty connections on gnd & positive cables trons dont flow well, sort of like a pinched off air hose letting pressure by but volume is down.
Or in case of trons volts are there, amps aren't look at the wires coming out of the battery big, look at wires in the alt small, and the single wire charging the batt also small, when you try to pass a lot of tron amps through too small a wire something has gotta give usually it is the voltage regulator section of an alternator.
When I have battery/charging issues I just check everything at one time rather than chase the dragon, batts-to belts-to alts-to gnds, make a day of it and go through your entire system.
It's easy enough job to remove and take for test, oh if you do find the alt is bad, make sure the pulley on replacement alt is same size as old alt as the tach signal on the dash comes from the alt. wrong size alt and indicated engine speed will be off. PCM does not care as it monitors speed from crank sensor in the engine.