So when you open the dump valve a whole lot of sewer water comes out or just a slight trickle. Have you left the cap off and with a container underneath does it start to fill? After I dump there is always some fluid left in the drain pipe that eventually settles against the drain cap.
Does the black tank valve seem stick or does it open and close smoothly?
Try this. Make sure gray and black valves closed. Put a 5 gallon bucket under the drain valve. Open the cap. Spray water into the opening until the water comes back in a good flow. It may be close to filling the bucket by now since you have to angle the bucket to fit under the drain. Once its flowing clean then leave the cap off to let it dry out then replace the cap. Come back a few days later and remove the cap to see if any black water comes out.
It could be the seals are dried out. You can add some holding tank chemicals to lubricate the valve seals.