That's a good question, Rough Rider, I've had the front panel to the base of the bathroom off in my HiLo, which exposed the piping under the shower drain, but I don't remember anything about it. (I was trying to mouseproof the trailer at the time.) I don't know your trailer's layout, but perhaps you could do the same thing and see what you've got there.
There certainly is very little drop into the grey water tank though.
- Jack