[Cyride]

I just use a hose end water meter, picked it up for about $15. I use it for a lot of applications including adding water to the black tank. If you don't want to buy one just put a gallon of water in your stool to get an idea of what a gallon looks like in it then just add that amount 5 or 6 times to the tank by flushing the stool. Not saying any other method is wrong, just what I found solved my problem. I don't get anything from "Happy Campers", a lot of folks on line recommended it so I checked into it. Check out their web site, kind of interesting (that is if you find reading about black and gray tanks and septic systems interesting).