Abstract: A sanitizer for recreational vehicles is interposed between the toilet flush valve and the toilet itself. It utilizes an upright canister which maintains a substantial air pocket to prevent back-flow, there being a charge of sanitizing chemicals in the bottom of the canister which dissolve slowly in the water that is introduced into the canister to flush the toilet. An inverted U-shaped tube introduces the fresh water into the canister, and by virtue of a vertically, downwardly extended length of the U-shaped tube which extends down into the canister, the sloshing of water into a downhill portion of the fresh water system is inhibited.The system overcomes the prior art arrangement in which a charge of chemicals is dumped into the holding tank with every tank flush, and it adds the anti-back-flow or anti-reverse-syphon protection of a three-stage prophylactic.