You might get one of these twist on valves to put on your drain hose connection. That way, you can fill your black tank with the normal valve open, add water to the tank and let it soak for several days, and maybe whatever is keeping your valve open will clear. It would also help to add a dose of tank cleaner that targets tissue, as that may be what's keeping the valve from closing.
http://www.valterra.com/product/twis.../?pcat_id=1473