The switch was located on the panel in the utility compartment, where the water pump switch and water service connections are located. The switch activated a fan that blew warm air down into the storage tanks area. As best I remember, it only worked when the furnace was on. My Newmar dealer told me they did away with the fan & switch in 2004; gravity now flows the warm air into the tanks area (which I don't think is as efficient).
'04 Newmar Scottsdale 3670
'04 Malibu Maxx toad (or '00 Jeep Wrangler toad)