Hello, we just got this motorhome and are trouble shooting the generator. He will crank like it wants to turn on but it won’t start. We even took a cup of gas and ran the hose from the fuel pump to make sure it was working, and it fired up for a second and then shut off. It’s almost like there is a switch or something stopping it from staying running. Is there a secret switch or something for this thing? TIA! 

Most ONAN generators have a blinking light on the starter switch that displays a two-digit fault code tied to uncommanded shut-downs.  Not sure about your model.  If you have the user's manual, check to see if does produce those codes. The manual should tell you how to read them and what each number means.