Richard,

When you get in a position that one of your tanks is almost full and can't dump it, you can always open the gray and black valves at the same time and they will equalize. Just make sure you have a end cap that doesn't leak.
Shut the valves after a few minutes.

They have the two tanks (blk & gray) because some state parks allow people to dump the gray on the grass.

In my opinion, they are both waste tanks, gray just sounds better.