I don't know the details of your particular coach, but I would guess the solenoid wouldn't be too far from the batteries and would have some heavy guage wires atttached to handle the amperage.
If both chassis and house batteries are charged, I would have someone activate the switch while I listened for the solenoid click by the batteries. It won't hurt anything to cycle the switch while you listen. If there's no click, then you'll have to find a wiring diagram and trace some wires to find the solenoid.
Let us know what you find.
Denny, Connie & Shadow (former barn cat made good)
2000 Itasca Horizon, Cat 3126B, Freightliner Chassis