My trailer has gen-prep but my setup is similar. I put the inverter and AGM batteries where the genny would go and then wired a 110v plug into the genny feed to the xfer switch. If I plug the cord into the inverter I'm off battery, otherwise I plug it into my portable generator. I also stuck a label on the inverter to tell me to turn off the converter. There's another label on the main panel where I mounted a lighted push-button that I use to remote on/off the inverter.
2011 Ford F350 CC LB DRW
2014 Eagle Premier 361REQS