A lot of people attach the extra valve to the end pipe where the hose connects. Then just open both grey valves and the tanks will equalize. You need an adapter on each side of the extra valve so you can attach it to the trailer and then attach the sewer host to the valve.
2011 Eagle 330RLTS with just about every option.
Dodge SRW 1 ton Cummins powered.
I was born with nothing and I still have most of it left.