Batteries wired and tested to have 12 volts output.
Look adjacent to the batteries for a fuse or circuit breaker
Go to the 12 volt power distribution panel, normally located adjacent to the 120 volt AC circuit breakers, check for 12 volts DC there. Look for a main fuse in DC distribution panel.
There should be a coach battery disconnect. Try cycling it to off then on. Listen for strong click.
There should be a large fuse for cables to the inverter, this is not the one. I would expect it to be a small one some where between 30-60 amp.
Look at manual about page 185 for battery disconnect information.
Manual