F53 chassis no power from chassis battery

2005 F53 chassis with battery testing at 12.75 volts all the way to where it enters the main fuse box. When I turn on the ignition I get nothing, no lights, no buzzer, no gauges, nothing. I push the switch to start with the house batteries and it starts right up and everything works. All stuff running from the alternator I guess after engine is running.

Any idea what is going on that I can not utilize the chassis battery? Is there a main fuse or something that has blown?

Not stranded or anything but looking for help.