2015 HD2500 Electrical/Battery Issue

[Ol'Jim]

Truck is a Duramax 6.6 diesel, crew cab 4x4. 78K Miles.
Have owned it for 6 months, No prior issues or indication of any problems.

Yesterday I went to start it, and it acted like weak battery, (it did start, but made a quick stuttering sound, and turned over a bit slower than normal).

As soon as it started, I looked at the dash voltmeter. It was a hair under 14V, (it normally ranges from just under, to just over, the 14V mark).

An hour later, it would not start. No click, the digital dash would go black, and all the gauges drop to the left peg. I turned key off, then back on, and voltage gauge showed approx. 12 volts.

It has dual batteries from the factory, original AC Delco units. Truck is 3 years and 1 month old. I put my multimeter on both batteries. The Auxiliary batt (drivers side front) read 11.93V, the main batt (Pass side rear) was at 12.12 Volts.
If this was a regular, single-battery vehicle, and since the alternator appeared to be putting out adequate voltage, I would tend to think the battery was dying. But the odds of 2 batteries failing on the same day, seem very unlikely. I don't know how the 2 are electrically isolated from each other, but they must be, because the voltage readings are different?

Had a buddy with a tow truck come over and tried to jump it, made no difference at all. Tried on both battery circuits. No crank, not even a solenoid click. He thinks it is the starter, but he is not a GM mechanic. My thoughts are that it is something else, because a bad starter would not cause the batteries to drain while sitting, right?

I went out this morning, and tried it again, same result, but the factory gauge shows lower voltage, (scale not marked between 9-14V).
My voltmeter showed both batteries even lower than yesterday: (Aux at 11.72, Main at 11.9). Makes me think there is a short or some kind of load on the electrical system, even with key off. I guess next step is to pull both negative cables, and recheck voltages? Thinking I might try disconnecting cables from both batteries, recheck voltages, then try putting my charger on one battery to see if it will take a charge? And if it does, maybe leave the Aux side disconnected, (in case that side has a short, since it reads lower than the Main batt), and try to start off the Main batt only?

Would really appreciate any guidance or advice from you fellas.

Thanks in advance. -Jim.

[Ol'Jim]

Ended up being the batteries. Replaced both of them with Walmart Ever Start Maxx units for $119.76 each + tax. 5 year warranty, first 3 years is free replacement, then prorated the next 2.
They are 730 CCA, 900 cranking amps. They are both date coded last month, 8/18.
Got them installed, and it fired right up.
Took a minute or so for the alternator to kick on, had me worried at first. Voltmeter read about the same with engine running or off. Restarted it, and needle moved up slowly to just over 14v, so all appears to be good now.

I think the reason it would not jump start, is because of the stupid steel/lead-free battery cable ends in use these days. They are so hard, the jumper cable clamps just can't get a good bite on the terminals anymore, so it is impossible to get much current through them.