Leaky black tank valve?
I found this little writeup and thought it was worth sharing...
The dump valve slides open and closes in a groove that has a rubber O-ring on each side of a nylon flat plate. What often happens is this: If you slide your valve closed while the tank is still emptying, toilet paper and solids can get worked into the groove--keeping the slide plate from making a proper seal.
Before you leap into an expensive dump valve replacement, try this easy procedure for cleaning the groove and O-rings. First, fill your holding tank completely with a mixture of water and dish soap (the dish soap acts as a lubricant for the rubber seals around the dump valve slide plate). As the water is emptying, repeatedly open and close your valve. This action works the solids loose and the running water washes them away. You may have to repeat this process a couple of times.