[Glenn and Laureen]

To follow up on this series of posts, I thought about what everyone said about how the plumbing is supposed to work, considered adapting my plumbing to add a discharge from the shower/kitchen line directly into the grey/black tank but decided to just let it work the way it was designed to function. I opened the valve to the grey/black water tank and let the water from the shower flow in. Guess what? It worked. No messy gunk coming up in the shower pan, no unpleasant odors, no bad anything. So, yea physics! Thanks to everyone for your advice.