There is a recent thread about this topic:
http://www.keystonerv.org/forums/showthread.php?t=23128
The OP of that thread had the same question as you, he called Keystone CS and was told one thing, other owners suggested CS was wrong, he called again and was told something else.
Moral of the story: Do some investigating, add water (one fixture at a time)at the different fixtures see just which tanks fill up and which are still empty, Do this by adding water and checking each tank by draining them one at a time until you determine which tank is filling with each fixture. Run water in the rear sink, see if the rear tank drains, flush the black tanks using the fitting, see if one, both or neither fills with water. You can do wonders to "understand your tanks" by actually doing some "detective work" on your own. You've already seen the results of relying on someone in a "front office in Indiana" and the "dealership's opinion" Now it's time to get "the Paul Harvey, rest of the story" on your own.