The Onan 4.0BGE generator crankcase ventilation system hose connects to the base of the air filter to carburator adaptor plate in order to recirculate crankcase vapors through the carburator intake. This hose connects to the valve cover on the backside of the engine. There is baffle, reed valve, and filter under the valve cover that could be getting weak allowing excess oil to be recirculated. It could also be due to worn or broken piston rings allowing excessive blow by (check by performing a compression test)
Get the spec number of your generator then use this page to determine the Onan manual number you need:
The RV Genset section is about 2/3 down the page.
You can get manuals from Onan, Ebay, or numerous RV dealers. Just google them.