Dicks advice is exactly what I would suggest. By back flushing the system with fresh water you might dislodge whatever is keeping the valve from shutting.
Now....the next problem is figuring out what that something is? If your RV is new, the most obvious something is the piece of tank that was cut out to mount the pipe with the valve. Of course it shouldn't be there, but it does happen. If that is the case, the only way out of the tank is thru the top by removing the toilet. That would be a real challenge to find the piece in the tank from that top entry.