A switch for this sort of power would probably be marketed as a battery shutoff switch, and may not really be intended to be switched under load. Auto parts stores etc. might carry such a switch.
However, if you have a solenoid based battery isolator, you can simply switch the control wire for the solenoid that connects the batteries. That's a much more readily available (and cheaper) switch, and doesn't require routing big heavy wires to and from it. The wires you need to switch may already be at the dashboard if you have an emergency start switch (depending upon exactly how the emergency start is wired on your motorhome).
If you have a diode based isolator, I'd suggest that you replace it with a solenoid-based one and proceed as above.
