Is this some kind of large manual disconnect on the batteries? If so, it is most likely something someone added to the vehicle to use for winter storage. It would allow a portable battery charger to be connected to the batteries without taking cables off.
Most relays/solenoids for the house batteries are mounted away from the batteries (corrosion and all of that stuff) and is controlled usually by a switch near the door where you can reach in and turn it on before you enter, or off after you step down from the coach.
Charles
