If a dump valve is defective, it's a pretty easy job to replace it.
However the cause of the problem should be determined to avoid a recurrence in the future.
Do you flush your black tank thoroughly when dumping, using a transparent elbow to ensure all waste material is out of the tank? Do you leave the black valve open when parked? Have you ever washed out the black tank with hot water to ensure there is no build up of solid matter at the bottom? Do you think the valve is frozen to the gasket>
When replacing your valve, spray the gasket with silicone, and once a year, disassemble the valve to apply more silicone spray.
To solve your current situation, try applying heat using a hair dryer to the valve to see if it comes loose. If not, try running some hot water into the black tank.
Please let us know how you make out.
Jimmy Dean Ross
1988 Newell Coach