I have a similar setup on my 04 Coachman. I bought a 4' hose exstention and took the end off so I could feed it thru that PVC swiveling pipe, attached the end to the Y fitting at the dump valves. Then I cut the excess of the flex hose off so I could leave this hose attached due to it being so hard to remove and install. I then reinstalled the coupling so I can connect the long flex hose to the sewer.
Hope this helps. One other note, I used the thickest flex hose I could find. It is one of the hoses that have red ends. I think it is Rhino hose but not sure.
Dan Sees, Full Timing in the MH & Part timing in a S&B In Hampstead, NC
2013 Winnebago Journey 42e, 2014 Featherlite Car Hauler 3110 17.5', 2008 Mazda MX5,
2008 Toyota FJ Cruiser