Sewer Tank Flusher

I fill the black tank through the toilet before romping. When using a flusher, I leave the valve open. The reason it is recommended to leave the valve open while using the flusher is the spray is much more effective spraying directly on the tank walls. If the tank is filled with water while using the sprayer, all you're doing is filling the tank and not really spraying the walls. You can test this theory by filling a 5 gallon bucket of water, sticking your hand in the bottom then spraying it. You'll feel a much reduced spray of water.