Normally Montana installs the Hepvo valve horizontally in the drain line line several inches from the physical drain itself. I am not sure how you can see it from the shower viewpoint. You will need to remove the backside wall of the pass through compartment on the passenger (door) side. That is the only access to it. The valve may be torn, maybe by someone previously trying to clean it with a brush or metal instrument. It is possible it is blocking the flow on water down the pipe. Look for this device.
