Winnebago - 2000 Adventurer. My black and gray tank valves are very hard to close. In fact, the black valve is partially open. I have added a valve at the drain hose. I have tried to lubricate the cables with no success. I have also tried to clean the black tank by several methods including with a turbo stick. Is there a way to get access to the valves so that I can lubricate or relpace them? Any advice will be greatly appreciated.