Your starter probably looks something like this
And if so it has a cheap plastic drive on it. From your words, it sounds like you are hearing only the starter spinning up. Correct? NOT the sound of the whole generator being cranked but not starting. Correct?
If I'm right in my understanding you have a good starter with a bad drive. But, on some of these ONANs, if you can jog the flywheel a little, a blocked or stuck drive may release and operate again.
That pic is from Amazon. I'm not sure the pic represents your starter. Somebody here can help you get to a service manual and a parts list. That said, the starters for several ONAN models seem to run between $35 and $55 on Amazon. If your generator design and installation allow changing the starter with the genset still in the coach, I'd replace starter with new.
That's if I understand your symptoms.
