Interesting; the build diagram shows a ball valve but it's located under the coach so there is probably a rod pushing and pulling the valve. Inside the compartment, it also shows a "T" handle (typical waste tank drain handle) for the fresh water tank drain. So, I'll assume it is a "T" handle with rod pushing and pulling the ball valve open and close.
So it could be a dirty stuck valve or it could be a rod that isn't traveling all the way to properly close the valve. Someone is going to have to get under the RV and do some maintenance.
