I rinse out my black tank with this. http://www.campingworld.com/shopping...on-valve/27868
This goes on the outlet of your RV. After you dump the black, close this valve, open the black valve and then the grey water. This should force some of the grey water into your black tank. Then close the grey and open the twist on valve and this should do a pretty good flush. Just make sure you have a lot of grey water so you have some left over to rinse your drain hose.
I have also used this when camping with no waste hookups to transfer some of the full grey water to my near empty black tank.
Its also a great emergency valve if you black or grey water valve were to ever fail.
All these uses make this item a must have.