Lazygranch, I'm no expert buuuuut this is what I found under my 3400rl. I had a blockage in my black water tank and had to start taking things apart. The area of the valves are just after the dropped belly by the drain hookup. First there were a few things to unscrew such as the straps that hold the lp gas line and a bracket that holds the dump line up. Then I removed enough screws to be able to pull down the underbelly and look inside. You may have to cut it along the dropped belly. It's screwed from the top. Carefully with a razor knife. Don't cut deep. There are a lot of wires in there. Maybe a foot + long. You should see the valves for the black and one gray tank. I think it's the kitchen water. As you know the valves are opperated with a cable. Have someone open and close the valves at the docking station and you should see the cable move the valve in and out. The cable may have come appart or the plastic housing may have broken. If you have to replace because of breakage, it's not made to come apart. What I did was cut about five inches out of the 1 1/2 inch pipe that connects two tanks. Then the valves can be serviced. I bought two 1 1/2 inch rubber plumbing couplings available at Home Depot or other similar store when I reassembled using the cut out piece. Taped the cut underbelly. Good as new.
Remember, I'm no pro and others may have better ideas.
Gluck