I have a 50 amp Lithium 2414 TH with2 AC units. I have a onboard Onan 4000 generator installed. I can only run one AC at a time with the 4000 watt onboard generator. Can I or how do I hook up an external generator so I can run everything with a internal and external generator. I was told I can run a 4000 watt inverter generator externally along with the onboard Onan. Please advise and thank you in advance.