Depending on the type of valve you have, there might be a rubber gasket that came loose from the valve. That happened to me. The gasket came loose from the valve and got "situated" in the pipe at just the right angle to keep the valve from being able to go all the way in. We were able to get it shoved out of the valve's location so I could close the valve, but we couldn't get it removed (and the valve was not leak-proof without the gasket.
Because I wanted to move the valve's position anyway (it was way underneath the RV), I ended up having the tank drained, then cutting the pipe near where the gasket was stuck, removing the gasket, adding new piping to extend the position of the valve closer to the edge of the RV. I also changed the valves to new ones that don't have rubber gaskets.