Looks like someone may have bypassed the battery disconnect switch.
The solenoid is usually used to temporarily tie the house batteries to the chassis battery for help starting the vehicle. There should be a button on the dash, that when pushed, the solenoid engages, and your starter now has power from all 3 batteries.
If the blue cable comes from the chassis battery, the red cable tied to it probably goes to the engine starter.
On the left side of solenoid, that cable may run towards the rear of the coach which would go to the 12 volt system for the house current. It would go from there to the generator for an electric start on the genny, (if it has one).
The upper right corner is an isolator. One cable comes from the alternator, and the other two cables go to both the house batteries, and the coach battery. The isolator will charge both the coach battery and the house batteries while you drive. The isolator will keep the coach and house batteries separate when the engine is not running, so if you discharge your house batteries, it does not kill the starting (chassis) battery.
Also appears as though someone bypassed the fuse in the upper left corner.