Have you done any diagnosis at all as to why the existing generator not producing power? Does it keep running more or less indefinitely with no power output?
In general, the Onan control modules will shut down the generator if there is no output from the generator head itself (as a safety feature--that often is because of a shorted winding somewhere, which if run long enough can get hot and cause a fire). If it keeps running, chances are pretty good that the problem is not the generator itself but something in between it and the rest of the RV--either the breaker on the generator tripped, or there's a loose or broken connection in the wiring, or the transfer switch has stopped working, or something like that.
If the generator shuts itself down, then it's still not necessarily a given that it's an overly expensive fix; it could be the regulator board or the control board or something like that. Proper diagnosis is the only way to find out. Calling a good mobile RV tech might be a good place to start.
Modifying an open frame contractor generator to work safely and properly in an RV is not at all trivial. The fuel supply needs changing, ventilation and heat removal needs to be considered, and the exhaust system would have to be modified to safely vent outside the RV, at a bare minimum...and likely there would be many more details involved as well.
