From the Onan troubleshooting guide:
CHECK BATTERY VOLTAGE Connect the negative voltmeter lead to GROUND and the positive lead to the
positive battery cable on the starter solenoid and read the battery voltage. The same reading should be obtained at
the BATTERY POSITIVE pin of the control board. A fully charged battery in good condition should read 12.6 to
12.8 VDC. Charge the battery if necessary.
CHECK CRANKING Try to start the engine. If it does not crank, check the control fuse (5A Slo Blo, except models
with electronic governor 10A). If the fuse is good, jumper the positive battery post to the coil terminal on the start solenoid.
If the engine cranks, there is a poor connection, wiring damage or the control board is defective. If the engine
cranks when the BATTERY POSITIVE pin is jumpered to the START SOLENOID pin on the control board, the board
is defective. NOTE: The battery voltage should not go below 9.5 VDC during cranking.