What about the Flush King everyone recommends? According to Camping World for $33 this is what you get:
Flush King Reverse RV flush valve provides you with the quickest and most effective way to fill, rinse and remove residue from your black or gray holding tanks.
Flush King rapidly refills the holding tank no matter what height of tank or angles in plumbing. Unique design eliminates messy wands and hoses inside the RV by allowing you to reverse flush your tanks from the outside. Powerful performance removes clogs in drain pipes and improves sensor performance for more accurate readings. Four locking hooks provide a tighter seal to guard against leakage. For your convenience, the handle swivels 360 degrees to make hook up and operation easier. The innovative clear barrel allows you to see when the tanks are completely clean of residue, so there is no guess work. Flush King fits all Trailers and 5th Wheels and most Motor Homes and is not a permanent attachment. Twist on and off, how easy does it get!