Like some others, we also wash our dishes in a plastic basin (in the sink), and then dump that water down the toilet. This helps to keep our grey water tank levels down and adds necessary water to the black tank.
Being it's just the two of us using the toilet, most of the time, we can easily fill the grey tank much quicker than the black tank.
Before we dump, I also like to bring up the water level in the black tank. I use a plastic door stop, like this one, and wedge it in the foot pedal that flushes the toilet.