Repair Valterra Cable Actuated Dump Valve

If  the valve is stuck open, it could be as simple as the cable being disconnected where it pushes the blade closed. If you know where the valve is, you should cut an access port about 24" square. Frame the piece you cut out and put a mating frame in the underbelly. Use 4 screws to replace the cutout and you will have an access whenever needed. Now, if the cable has pulled out you can fix it without any need to open or change the valve itself. The cable fits into a hole in a cylindrical collar on the rod that pushes and pulls the blade. It is held in place with a set screw. If it is not closing because the sheath that the cable runs in is pulled out of the plastic housing of the valve, you will have to loosen the top two screws to reseat it. It fits into a slot with a shoulder which you will not be able to see unless you take it apart but you can push it in until it stops. Visit Valterra's website and you may get a better idea of what it looks like.  It would also be helpful to go to a place that sells them and have a hands on close look.