Black water tank light says 2/3 full after draining

When we use the dump station I have a clear elbow to see when the black tank stops running. Then I completely fill and flush the toilet bowl twice (about four gallons) and check that the water is running clear. The tank is as empty as it is going to get.

Same with the gray tank; dump, fill sink twice and look for clear water. Then I know that it is empty also.

The gauges rarely read empty, especially the black tank.

I also like to keep 5 gallons or so in the tanks so they do not dry out once they are dumped.