I have the same switch on my '99 Minni Winnie and have yet to find what it is for. I do have an idea though. I think it might turn on the outlet in the outdoor entertainmment center. I have not checked it out though. I will be glad to see if anyone knows exactly what it is for.
In my old 98 Itasca Sundancer a switch in this area controlled the courtesy lights near the floor in the hallway. Took us a few months to figure it out. We kept trying the switch during daylight (hint: wait till dark and see if anything comes on/goes off).
I visited with my friend this morning and he has found that the switch controls the Courtesy Light at the Bathroom Door.
He is very appreciative of the help given by the members here and the Rep at the Winni Help Desk in Forest City. I also Thank You all for posting.