Phil, are you talking about the switch that has what seems like a Rectangle with a serrated line down the middle, and one little "stem" line on the right side? I have one of those and don't know what it is... mirror heating is certainly possible!
Sadly, I don't have a coach manual; I keep my eye on ebay to see if another Summit comes up for sale locally where I might ask the owner if I could copy the manual... no luck so far.
The Green Machine -- 2000 Mountain High Coachworks Summit (Spartan chassis / Cummins ISC)
...and F-Troop: Fearghus, Fiona, and Frankie (Cairn Terriers)