Depending on the model of generator the engine is likely shot. If it is the Onan KY series (generally 4000 watt and under), they have a typical service life of around 2,000-2,500 hours. If it is the 5,600 watt or larger gasoline models they tend to last somewhere in the 3,500 - 5,000 hour ballpark, assuming regular oil changes and maintenance.
As to what to do, if it were mine I would be shopping for a pull out Onan, lots of RV generators get very little use, therefore it is fairly easy to find a 15-20 year old pull out Onan from a coach that is being scrapped that is the same as your old one for $750-$1,200 in good running condition, or often half of that if it has issues like gummed up carburetor or a bad control board.