Is this a cable or pull rod type of valve? If it is a rod, I'm pretty sure the rod is just threaded in. Can you see this valve? Can you see that the rod is disconnected. Can you pull the handle all the way out. The reason I ask is because nothing comes out doesn't mean the valve isn't opening. Black tanks are notorious for this. If the valve doesn't close completely the liquids leak out leaving the solids behind. Get a King Flush and put that on the end of you sewer connection. With the black valve pulled, hookup a hose and turn the water on. You will be able to tell if the valve is open or not.