It sounds like the cranking RPM's are low. Could the starter have intermediate issues that would cause this? I checked the batteries and they both showing good charge.
First, yes, the cranking RPM is critical. If the PCM doesn't see at least
100rpm it won't fire the injectors and you'll never start (normal is @150-200).
Second, "showing a good charge
" ? What the heck does THAT mean?
There is only ONE way to test these batteries. That is, charge them up fully, have them individually Load Tested at a High Rate (not those dinky 100A load testers the Auto Parts Stores use on their ricers). You need them loaded at 250-300Amps, these are high capacity batteries facing a Huge load when trying to turn over a high compression diesel while running a 150A draw from the Glow Plugs...
If the batteries aren't up to spinning the motor fast enough, and when the Cold Weather arrives, they'll become useless, then you need to know now. The Load Test will tell you that.
In either case, this is the time to go over all the battery connections anyway. Clean, Scrape, Grease & Tighten ALL the battery cables, BOTH sides. Then, if all else is good, you might consider the Starter or even the Relay maybe...