Didn't see a followup from the OP, with a diesel, get a battery tester, one of the $20 resistive ones works fine.

remove and clean every cable connection, batteries, grounds, starter. Charge each battery seperately and test each one.

If you have just a click, it could be the upper starter solenoid, look for voltage with a test light on the outbound side when the key is turned to start. If you have voltage, your next stop is the starter itself.