Originally Posted by Wild Irish
It is starting up just fine, as long as I hold down the start button, but as soon as I let go, it dies.
Almost all Onans are designed to bypass all the safety cutouts during starting. When you release the start button, they are reconnected and if everything isn't right, the engine is shut down. Your problem is very common. It's a sign that a sensor is telling the Onan something is wrong, but exactly what will vary. It could be the oil pressure switch. It could be the absence of 12 volts. It could be the absence of 120 volts, it could be a broken overtemp safety sensor. There are other possibilities. Don't spend too much time running it with the switch held down, in case you have low oil pressure (I'd check your oil level first).
If you post the model number and spec code from the data plate, someone can help you find a manual that will have a troubleshooting chart.