On my '74 26' Argosy, both the grey and black water valves pull out to dump, no twisting. It has a T handle that when closed there are wire loops that hook over the handle to prevent unscheduled openings. There was no telling how long it had been since they were operated. I put enough water in both tanks to make sure the valve seal was in liquid, thinking it would help lubricate the seal a little. Then I just started by pulling the valve hand with some amount of force, but not so much that I thought it would break. If it moved a little, I would push it back in, then out and in until it was free and would move the way it should. At times it took a little tapping to get it to go back in. I tapped gently until it moved.