Is it the solenoid on the starter or the relay on the fender (the one on the fender is a relay) that's clicking?
Try putting the batteries on a slow charge. Separately if possible, but not critical. Then see if it'll start, and if so, wait 'til the glow plugs cycle off and check the voltage with the engine idling. It should be 14-ish volts. One simple test to see if it's a parasitic draw or just batteries that will no longer hold a charge - after you shut it off, disconnect both batteries and check the voltage. Should be 12.4-ish. Make sure they're separated. Then leave the truck and come back check again. If voltage has dropped on either/both batteries, they're the culprit. If they're about the same, then it's more likely a parasitic load.
Do you have a battery load tester?