Hi PaMa,
There should be a switch inside the coach, probably with all your light switches and other controls. It's usually an illuminated wall-type light switch. Page 3 of our
Water Heater Usage Guide has a picture of a control panel from a Landmark, where the tank heater switch is right in the center of the controls.
When you switch it ON, there's power to all the heating pads on the fresh, gray and black tanks. It's important to keep a little water in the tanks. If you run the heating pads with empty tanks, you can burn a hole through the tank.
It's possible you might have a different type of switch; possibly one marked YETI. Sometimes with the YETI option there are 2 switches; one for tank heaters and another for the heat tape on the fresh water feed line. Implementation varies a bit by model.