[A/S] Mystery switch in bathroom

[<nudog@xxxxxxx>]

Have a mystery switch in the bathroom of my '79 Safari, rear bath unit, 29 ft.

As you face the rear window in the bath, standing in front of the commode, 
there are three switches to your left, between medicine cabinet and the window.  
Two are labeled, on for the light above the sink, high and low, one is 
labeled water pump.

The topmost switch is not labeled, and cannot find a reference to it in the 
owner's manual or service manual.  Anyone know what this switch is for?  If I 
turn it on nothing seems to happen.  There is no bathroom vent, no light in the 
closet goes on, or anywhere else,  

Anyone have a clue? 

Thanks, Jim