Yeah, it is always helpful to include exact year, make, model of the rig in the discussion. We have just over 100 hours on our little diesel generator in the 18V24D, no issues at all. Usually the Cummins/Onan shops are pretty good at diagnosis so I would be on the phone to WGO, make sure your service center is talking to them and get it done. It could be the Onan shop is correct and as posted above it could be a simple fix. Hard to believe WGO did the return line wrong, easy to imagine a kink or other line issue.
Bill and Brenda + Mobius Big Dog
2018V24D, '13 Tiffin BR32, Tiffin 34TGA, '11 Aspect 30, 06View23H, '00 HHiker II 5W