Your tank heater is a square patch on the exterior of the tank(s). And yes you should have some water/liquid in them to use. Additionally, you should have wrapping (a small strip of heat tape) around the pipe(s) leading or next to the dump valve(s). I have used mine for every winter (7 yrs.) and have left the switch on with no damage. This switch turns on all installed heater patch(s) for your tanks and drains.
Don & Dona W/yorkies Gizmo, Master Chief, & Tali
2011 Flagstaff 8528 BHSS 2015 Ford 6.7 XLT
2010 Full Timers & Still Going