Reviving this thread as I've had a good bit of time to diagnose this. Still haven't solved the issue but here's a bit more information about what's happening to me in case someone else is experiencing this:
I feel like it's something to do along the path between my batteries (dual Optima Yellow Top's with 800CCA each) and the cold start timing advance system. Here's why:
Pre-turning her over:
When I first turn the key over partway to allow the glow plug relay to come on and the "wait to start" light is on my voltage is typically in the 10.5-11 range which falls in the "good" starting range on my meter. However, sometimes I read a voltage closer to 12-13 at this point (no idea why sometimes it's different). BUT If I wait long enough with the key in this position the voltage will almost always jump up (guessing the second battery kicks in at some point?) closer to the 12-13v number. Sometimes it takes 20-30sec for this to happen but it almost always does if I wait long enough. Anyone know why there's a delay happening here? What controls the flow of voltage between the batteries and the glow plug relay/cold start timing advance system? It feels to me like I'm initially getting just one of the two batteries involved and then the second one joins the party after waiting long enough but I don't know why I'm getting that second voltage bump after waiting... I also have an amp meter and during this whole process my amps are drawing from the battery which is to be expected at this point.
Turning her over and running:
After I've waited for that second voltage bump I turn the key the rest of the way. As she's turning over and starting my voltage is typically right around 11v maybe 11.5v which is just fine. Once started and idling under the influence of the cold start timing advance things look good at this point. I'm at 14v and everything is peachy (my amp meter is showing positive input into the batteries) until the cold start kicks off... Once that kicks off and the idle drops I get a big reduction in voltage back down to starting numbers (11v or so) and I'm drawing amps vs adding amps. Sometimes things kick right back over but most times it takes anywhere from 5-30 seconds to get my voltage back up to 14v and positive amps back into the battery. This behavior happens regardless of driving, idling or coasting.
My thinking is somewhere along the lines when the cold advance kicks off my truck somehow thinks we're back in a starting condition vs driving condition. Definitely not willing to rule out the alternator but the erratic voltage behavior while starting leads me to believe the alternator is not the most likely candidate here. Anyone have an idea on what might be going on or even good information on wiring diagrams for the starting, glow plug and cold advance timing systems they might be willing to share?
Apologies for the novel too but I feel like precise and specific information helps more vs just saying I'm getting weird voltage behavior