It will take quite a bit to have some cover the entire bottom IMHO.
I have one of those rv black tank flush devices that allows you to back fill the tank with water from the outside. Because of the way the tank pitches towards the outlet, it actually takes a good minute or two before you fill the outlet pipe with water. Then...and only then... will water actually be making it's way back into the tank.
One gallon added to the tank will most likely all flow down towards the dump valve and I would doubt that a typical drive would slosh it back into the tank enough to do much good.
