Open generator shroud on the ONAN cover (with the two flip latches). You can reach the flywheel pretty easily from the left side; it's basically in line with the vertical baffle in the generator case.
Check the voltage at the generator when trying to crank, and in particular across the starter solenoid. It may be as simple as a bad connection there, or a loose control wire on the solenoid. Hopefully it's not the starter itself or a connection to it as you need to entirely remove the upper generator case, which generally means dropping the generator. On my Coachmen, it took me a good portion of a day to disconnect the necessary connections and drop the generator (to change out a broken starter).
