Neat. Something I have had on my list of things to do. Two thoughts, I did something similar to this on our first trailer, but I added a one way valve to the black tank flush (attached to the 1/4 turn valve).
For your cable hook up Radio Shack has right angle adapters that are push on, no threads. I use these, makes attaching the cable a whole lot easier. I keep a couple in the trailer, as they eventually get a little weak (spring clamp is thin).