[Mikendebbie]

Just to make sure what you are seeing...
1. You have added a valve where your sewer hose connects to the unit
2. Your OEM tank valve is not shutting off the flow from your tank because you are seeing liquid backed up behind the new valve at the sewer hose connection.
3. You think there may be a leak somewhere in the plumbing above the coroplast bottom...you describe this leak as “inside the undercarriage cover”. I take you see liquid leaking out somewhere from the black coroplast.

If I am describing your issues correctly then item 2 and 3 should be covered under warranty. The valve you added at your hose connection is working properly. That valve prevents lots of “surprises” we have all experienced from removing the cap to attach the hose at the campground.