Originally Posted by rumbleweed

Ok you don't say what TM you have, so I will assume that you have a model with the Thetford recirculating toilet. There is no black water tank in the traditional sense. The toilet itself is the black water tank. It does not empty into the grey water tank. To drain that tanks follow the procedure below.

Make sure both gate valves under the TM are closed.
Prior to closing the TM pull the "T" handle near the bottom front of the Toilet.
at the dump site,
hook up the hose to the TM and dump station tank
There are two gate valves under the TM one in a large line and one in a smaller line.
pull the "T" handle on the larger gate valve. This dumps the toilet.
Once it has drained,
Pull the gate valve handle on the smaller line from the grey water tank.
Allow that tank to empty it will also rinse out the dump hose some
close both gate valves and you are on your way.