My batteries where both on the weak side sometime ago. I'd put them on the charger for about an hour then turn the charger over to "start" and it would just barely start. I drove over to the closest Auto Zone, bought two new batteries, leaving it running while I was there, got home and changed them out. Problem simply no longer exists. It seems, even though there are two batteries, that these diesels seriously require proper voltage and draw to start.
Ken....