Banshee365,
There are a variety of "black tank flush" kits available at any RV center. I installed one on my Springdale. It was a "spin spray" terminal that bolted into a 5/8" hole in the side of the tank, a 10' section of clear plastic hose and a hose connector/anti-siphon valve that mounted through the J channel near the bottom of the trailer. I think I paid around $15 for the kit.
If your trailer doesn't have the black tank flush connected to both tanks, you can modify the "missing tank flush" very easily.
http://www.campingworld.com/shopping...nnection/28091