Lewster and Protag are the experts in this area. I'm a Newbie. That said, as an EE, there is one thing that annoys me when we talk about the Magnum. There are Rectifiers and there are Inverters. They are not the same thing. The Magnum is a Rectifier AND Inverter with an AC Transfer switch. The Rectifier takes AC power, converts it to DC to charge the batteries. It is a separate internal circuit. https://en.wikipedia.org/wiki/Rectifier
. Then from the DC bus (connection) takes DC and "Inverts" it to AC. The circuits are separate.
So for trouble shooting keep this in mind. Are the batteries being charged? Yes/No. Is the Magnum producing AC yes/No. Is AC passing through the AC switch? Yes/No. Answering these questions can help isolate the area of the problem. Then as Lewster suggests, look at the settings of the Magnum and make sure they are correct as they could be asking the Magnum to do things that is causing the problem.
I know very little about the ONAN Genset so I'd follow Protag's advice on checking this to confirm or rule it out. But if the Magnum works flawlessly on the batteries then that is a very good clue. Also, you have a 220 amp alternator that is charging the batteries down the road so why run the Genset unless it is so hot you need the House AC.
Good luck and let me know what you find out as I have a '16 as well