black tank mistake, not adding water after draining?

Although it is no fun and you MUST wear gloves, you can use a Q-Tip to clean the groove that the ball valve slides in and that will (SOMETIMES) cure a leaking seal.

If you have to change seals it is not a hard job. Easier with two people to handle the bowl lift-off. Order the seal kit on E-Bay and it will be about half of what a RV parts place wants.