The screen shot below is from a 2001, but yours is probably the same. The item on the left is your tank monitors - grey, black and fresh water. I would guess that the switch on that panel is for the tank heaters. I am guessing at these - maybe someone else will have better info.
Of course the other items will the vanity light switch and GFCI outlet.
Mike in Austin, Aggie ‘77
2018 Montana 3921FB
2019 Chevy 3500 High Country DRW