Our original Onan Generator tries to start.. sounds like dead battery. Which battery is this starting from ? House or Chassis..
All systems good with fuel, oil but slow cranking a couple of revolutions and stops...
It came from the factory wired to the house batteries. Someone could have changed it.
Check the voltage at the solenoid at the generator. It very possible that the starter is bad and
Check voltage when the start button is activated ?
Check before trying to start and also while the start switch is activated. That should give you the answer that you need.