If I was a guessing man.... I have to put my money on water pump...
My reasoning is... back in '82 or '83 I installed a switch just like that one for an older couple who didn't want to have to walk over to the wall mounted pump switch every time they wanted to wash a glass or get a drink when dry camping...
But it could also be for the electric water heater since Suburban makes you walk outside to flip the switch...
