There's a solenoid on the starter. When you turn the ignition key, it provides power to the solenoid primary coil, which engages the solenoid and connects big high current cable from the chassis battery to the starter motor itself.
The primary coil doesn't take much power to engage from the ignition switch, it's typically a little thin 15/20 amp wire. The secondary wires on the solenoid however, are meant to carry a couple hundred amps, and put a HUGE load on the batteries.
If batteries are discharged with low voltage, you may have enough power to engage the solenoid coil, but no where near the power required to spin the starter motor. This is the click you hear, the solenoid engaging. Under normal operations you don't hear this click, because as soon as it clicks the starter motor starts turning and engaging the flywheel, making a huge noise starting to turn over the diesel.
Your symptoms are consistent with a discharged chassis battery.
"Why" your chassis battery is probably discharged, still has to be determined as I mentioned before.
With your chassis batteries 'at rest', not having any charge current given to them in prior half hour (whatever your coach uses as a chassis battery charger), what is their voltage as measured with a DC digital voltmeter?
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DaveB, Raleigh, NC
2015 Tiffin RED 33AA, w/Honda CRV
VMSpc, Magnum BMK/ARC50
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