If the starter is spinning fast enough, it should throw the drive gear out.
If the drive gear is sticking, it may be because of the gear being damaged.
There is also the possibility that the windings in the starter are going bad, and the starter simply is not spinning fast enough.
The starter shaft has a spiral groove in it that the drive gear slides out on. The chances of the shaft being damaged are minimal, but there is a chance.
Replacing the drive gear assembly on these starters is much cheaper, but is also an awful lot of work.
Honestly, if it were my own set, I would simply replace the entire starter - due to the difficulty in changing the drive gear, and also due to the age of the set.
With a new starter, you know you are not going to have to worry about it.
If you need help with part numbers for this, please let me know.
This starter is no longer available through Onan, but is available in an aftermarket version.
The starter is part number(NNN) NNN-NNNNand runs $136.06
If you wish to take your chances with the drive kit, it is part number(NNN) NNN-NNNNand runs $24.95
Both parts are available here: