Through our phone conversation, and Peggie's direction, I went down and took a more careful look at things... and then made a drawing.

Turns out that I may be quite alright in several areas. I am plumbed to the lav drain and use the sink as one "vented loop" and I have a vented loop on the output side.

This made the whole system quite easy to fresh water flush... Which I just did.

Still need to figure out a way to empty the tank beyond the 3 mile limit.

I'll post more in a later thread.