A few questions
On prime does the fuel pump hum like normal?
Checking 12v input is fine what about output to starter?
Can u take out plug to release compression maybe reach in deep an bang on starter?
I'm thinking starter the bendix on the Onan is plastic trash that locks up an falls apart a current meter on the output should help.
I had a bad starter that would work half ass then finally nothing the starter needs to wind up to throw out the gear into the flywheel once locked out nothing happens and seems to go when you least expect it.
If you don't have a slideout Gen it sucks to get to the starter as it's on the back side.
Another option is to see if you can gain access to the fan an turn the flywheel it may unlock the bendix you may get a few more starts but will indicate its time to change the starter.