Thanks for all the creative and entertaining suggestions as to the purpose
of my mystery switch.

See photo here: http://www.takingflight.net/images/switch.jpg

Some things that I've checked out which it is NOT:

It doesn't affect the running of the refrigerator,
Or the fan on the back of the fridge,
Or the control light panel under the range.
Or whether the cigarette lighter power outlet at the front of the camper
cabinet has power or not.
Or the sink pump.
It doesn't matter whether the ignition is off or on.  All of these things
are live and are run off my auxiliary battery.

I haven't followed wires because they're hidden inside the cabinet and I
don't know how to remove the top, and don't really need to learn to, except
to satisfy my curiosity about this switch.

Perhaps it will remain a mystery!

Thanks,
Paul