Some Onan genset did charge the batteries, and we're not dependent on the converter, example back in the 70's
idk what model year they made the change and removed the charger section
To the op
Remove the brushes and clean them, a little smoothing on the sides with fine emery paper, you just want to keep them from sticking
When I worked on motors and generators we used a cut to size piece of hardwood to burnish rings and comutators
The field brushes and slip rings in a genset are low voltage low amperage
The seldom pit or burn unless sticking in the holder and not making contact
Sounds like the fuel pump latch circuit is not working
As a test you can put a jumper wire on the fuel pump making it hot and running
And see if the generator stays on
Also check the low oil switch circuit
You really do need to use it more often
With generators the old adage "use it our loose it" proves to be true most of the time
