2015 Peterbilt 579 PACCAR MX-13 - Intermittent No Start / No Crank

I have a 2015 Pete 579 with the MX-13 that started acting up last week. Sometimes it starts fine, other times I turn the key and get absolutely nothing—no click, no crank. The dash cycles and I see a P0607 (Control Module Performance) code stored in the ECM. I’ve checked the batteries and they are holding 12.6V. Terminals are clean. If I jump the starter solenoid directly, it cranks over and fires up, so the starter itself seems okay. It feels like an electrical handshake issue between the ignition switch and the CECU or the ECM, but I'm not sure where to start digging without tearing the whole dash apart.