Robert,
You can access the grey and waste valves only by removing the sheet metal fascia panel. The key to removing the panel is to remove the rear gas prop and support the compartment door with a dead man stick in the fully open position. The rear upper comes out first then move the panel around to gain clearance. It will tilt out enough to see the dump valve mechanisms. Some coaches have enough of a service loop in the plumbing to do the repair. On my coach, I ended up extending the plumbing hoses necessary to gain full access to the compartment. Because of the effort involved to get that far, I would recommend replacing the dump valves with new ones. Hope this helps, Fred