2006 Landmark - How do I access the dump valves

I have a 2006 Landmark and the pull handles for the dump valves are in a docking closet. I think I have a problem with one of the valves because the pull handle cable seems like it is binding and doesn't seem to work very well. But for the life of me, I don't know how to get at the actual valve. There is a small hatch (very small) in the docking closet but no way to get to the valves through there. Can someone please tell me where the valves are and how to access the them from underneath?

Thanks