Black Toggle Switch
I had a 2002 Sun Voyager, and it seems to me that the switch you refer to is the water pump as it is located in reach of the sink area. I had such a switch on mine. Look at your systems panel when you turn it on and if it is the pump switch a light will be on indicating the pump is on. There also may be a switch in the bathroom.
2005 Crescendo triple, 2000 Blazer toad, Brake Buddy,DW,2 cats and lots of patience