I come up with Onan number 185-7269 as an Onan alternator
at near $750. Is that what you're looking for? If so. it's not necessary to replace it with an Onan one. Almost any 12 volt, internal regulated will do. If you only need stock charging
amps, there are several on Ebay. Some under $100. Even if you have to hire a marine
electrican to replace it, you'd have money
Another option is have a rebuild
the one you have. Still way less than $750.
With a picture we can probably id the manufacturer. Delco (GM) are common on many engines. The picture I have under that number looks like a 10si.
You could have a problem other than the alternator
. Most auto parts
stores can check alternators, usually free.