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I will have to be very general because I don't know anything about your particular unit. The winterization kit (BY PASS) valves will be in the cabinetry behind the heater. They will be 1 or 2 or 3 quarter turn valves depending on the type of valves used. In some fancy trailers they are in a compartment, either luggage or a specific utility/water/dump door on the outside of the unit and slightly removed from the heater itself. Some units have a electrical valve set operated by switches hidden behind a decorative panel inside the unit. Finally a failure to fill could in theory be a bad check valve at the top outlet of the water heater, but I doubt that as the you havent found the other valve or valves yet. Good Luck