2006 Peterbilt 379 will not start

I’d check each battery with a voltmeter. I’ve never seen it myself, but I have heard of new batteries having the polarity reversed.
Have you checked for fuses behind the main instrument cluster?
I have a 94 and there’s a 5 amp marked ecm back in the wiring harness.
@Big Road Skateboard @Goodysnap

 

Had the same issue at work with a ‘93 375 w/ISM. Mistakenly took all 4 batteries out, then a couple months later put 4 batteries back in. Engine turn over great, but like yours, no start. Crank, and crank, no smoke from stack. Had two different mechanics look at it and checked all wires, etc. Never did figure it out. I have since retired, not sure if truck is still there.

 

You’re not gonna want to hear it but it’s an ECM power issue it’s loosing voltage when your key is in the crank position. Your tested voltage yes but not load tested the circuit. See if your ecm power and ground will light a halogen headlight.

Either is never a good thing but I have had customers bring trucks to me that they thought had fuel leaking down. They would crank on either then run. They wouldn’t couldn’t accept it being an electrical issue till I sent it home running. The problem began when you were meddling around in battery box. That’s where the problem is

 

If all circuits hold a headlight load and shakedown it’s new ECM time.

 

Hey @wore out, I may be thinking of E model, but did the C15 have a ground stud on the left side of the head?

I vaugely remember having a couple no starts with issues at that ground. Maybe I'm mistaken, or not C15. Long time ago.

OP, I would pull all those batteries back out, and would be surprised if there's not a wire you missed

 

Yes the ECM has an internal battery for Keep alive memory. A fault will typically log active for the internal battery.

I would also suspect wiring first. Shake down wiring from the ECM to batteries for that sealed beam to flicker or dim. Also check while cranking.