Hey Bad Moon
We think alike - putting the add-on valve at the drain pipe provides a safety "valve"(pun intended), and keeps your feet dry! I put it on last summer and it saved us this spring when our black tank valve was jammed (frozen) and the cable came lose. I was able to use the extra valve and make the trip.
I also do the same gray tank back-flush for the black tank every time we dump. Works great. And the clear elbow is a very smart add-on. Might be a little gross looking, but it really helps you know the tank is fully flushed.
Link for waste valve on Amazon - $23.
https://www.amazon.com/Valterra-T58-...ra+waste+valve