To prevent sink water from running down the drain, we bought these tubs that fit perfectly into the sinks to catch the water. Generally, this water is just from washing your hands or rinsing something off. Once full, you can dispose of the water in another way.

This keeps our gray tank pretty dry... now if I could just figure out how to do this with our Black Tank.

I've posted this before, so if you're reading this for the 2nd time, then I apologize.