I highly recommend one of these
http://www.campingworld.com/browse/p...rs/prodID=3709.
You can watch what is coming out of your holding tanks (or recirc toilet) and the gray tank to verify when it is empty, and whether you want to flush clean water through again to clean the tank(s).
The best information given above is (1) keep all your valves closed until you are ready to dump, (2) dump the black tank first, (3) dump the gray tank last.
Happy trails.