An improved seal element for use with a self contained recreational vehicle sanitary system. This invention is particularly adapted for use with the type of sanitary system having a bench structure which defines an interior storage compartment and a waste holding tank removably disposed in the storage compartment. Waste products are transferred from the bench structure to the waste holding tank via a fill opening. A seal element is disposed around the perimeter of the fill opening which seals against both the bench structure and a movable blade valve mounted within the holding tank which closes off the fill opening when it is desired to remove the holding tank for dumping etc. The seal element includes one or more sealing lips which engage the valve blade member in its closed position. One of the seal lips is disposed in a generally radially outwardly projecting direction with respect to the fill opening when the valve is closed. This seal lip is self actuating since it more tightly seals against the valve blade in response to internal hydroplastic pressure thus providing superior protection against leakage of the contents of the waste holding tank.