***Need help with a 7000kw gas Onan generator***

Hi guys. New on here and need some help. It's on a Onan model 7HGJAB-900C Spec c. This is on a race car trailer of mine. The gas generator starts right up and has AC power but as soon as I flip on the breaker on breaker panel on the trailer I lose the AC power??? Any suggestions on what to try?