12 Volt Power Issue

We have a Forest River 2015 Legacy 340BH.
Our chassis batteries were low, so just started the generator and held the Aux switch while trying to start the coach.
Within a minute everything went dead.

Installed new chassis batteries, and everything is working there, however no 12Volt power to the coach except when the engine is running or we’re holding the Aux switch.

At that time the disconnect switch on the wall in the coach works too.

If we turn the ignition off or let go of the aux switch, the disconnect switch on the wall doesn’t work and we don’t have 12 Volt power in the Coach.
In addition, connecting to shore power doesn’t provide 12Volt power in the coach.

We’ve also verified the coach batteries are fully charged.
We’ve checked everything we can think of and feel fairly confident it’s a blown fuse but have not been able to locate it.

Any suggestions would be gratefully appreciated.

We do have an appointment in early February if nothing else.