A dual voltage auxiliary heating device designed to be applied to the exterior of a holding tank in a RV or camping trailer. The 12 VDC circuit allows the tank blanket to be used underway on the road, while the 120 VAC circuit allows the tank blanket to be conneted to a 120 VAC power source during long stationary stops. One blanket is designed to protect a 15 gallon tank from freezing. The thermo-switch in each heater circuit turns on at approximately 40F(+/-5F) and turns off at approximately 60F (+/-5). This allows the tank blanket to add warmth to the tank as the temperature drops.