There will be a place in your docking station labeled something like 'Tank Flush' to hook up a hose, do not use your fresh water hose. Look at the pic below and you'll see the white water hose. Next to it is the green hose for the black tank flush.
On second thought if you have one it might not be in your docking station. After thinking about it our previous Jayco TT had the black flush connection on the side of the camper but it was labelled.