As said, the fullway termination valve is another name for the black tank 'blade type' drain valve.
The disclaimer is there to hopefully prevent you from flushing the black tank with the drain valve closed. Should you get sidetracked while flushing, the tank could fill, bust, squirt crap (literally) out the vent pipe on the roof, or leak inside around the toilet.
2016 Flagstaff 8529IKBS Diamond Package 5th Wheel
Ford F-Series SCREW 4x4
TST Tire Monitor - Sinemate 3500w Gen.
2016 Nights Camped = 42