I have the same generator as you and I also had the same problem. I took it to Onan for servicing and at first they thought it was a valve cover gasket that was leaking - a rather simple fix - but, it turned out to be much more serious and required that the entire generator be removed from the coach. The problem, a leaking oil pan gasket. Total cost of repair, CDN$1062.00. Ouch! This unit had 250 hours on it when the leak first appeared. I am also meticulous about the maintenance of the unit.
I wonder how many other HGJAB owners are experiencing oil leakage?
2002 Triple E Signature A3607GW
300HP Cat 3126E, 6Sp Allison