I found that the stock alternators on my Onan gensets were nearly useless for anything more than maintenance
on a small starting battery
. I disabled them and permanently used the "shore power" charger
to charge all the on-board batteries
whenever the genset was running.
So if the alternator has just failed electrically, don't spend money
on replacing it.
If it has failed mechanically then you may need to fix that enough to get your belt running.