I am extremely familiar with this exact symptom (motor starts but shuts down as soon as the START button is released).
Depending on the exact production model of your Onan, it can be one of two general issues. If it's an older model, Onan used a oil level sensor to (in theory) shut down the motor if you run low on oil. If CW changed oil, I would check that first. Too low or too high on the oil level will prohibit the motor from running. Note this is only for older models; Onan no longer uses the oil level sensor.
The other possibility is that the generator is not generating proper voltage, which again prohibits the motor from running. That was the problem with my Onan, after 500 or so operating hours one of the generator brushes failed.
I highly recommend Cummins-Onan service, if your oil level checks good. They are extremely professional and I'm happy with their service here in Orlando.
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