My bathroom sink dumps into my black tank. Whenever I have extra water I dump in into the bathroom sink.
I have a small plastic tote. If I need to quickly wash something up I do it in the tote and then dump that in the bathroom sink.
After boiling some corn on the cob I dump the water into the bathroom sink.
I'm just a weekend camper. I almost always camp at places with no fresh water hook up. So I use my fresh water tank. I try to put as much of that water down the bathroom sink as I can.
That is how I equalize my tanks. Just put the same amount of water in both of them.