Got a multimeter? First test is the testpoints on the power panel. Voltage between ground and the 12V TP and ground and 24V TP. 12V is hot when batteries are connected, 24V appears when the ignition switch powers up relay K2. With the switch on the testpoints should both be the same as battery voltage between the 12 and 24V battery cables to ground. Around 12.6V and 25.2V respectively if the batts have a reasonable charge. If you are not near these voltages, charge the batteries.
now observe the voltages at the two test points while you try to start it. If one drops significantly, look back thru that circuit.
just troubleshot something similar on my truck last week, turn on switch, warning lights and low air buzzer come on, push start, starter goes clunk and everything goes dark. My 12V was disappearing at the power panel when I hit the starter button, which shut of the 26V being provided by relay K2. Traced it back to a bad connection at the reverse polarity diode module below the airfilter outlet. Cleaned the connections and she started right up.
Good luck.
