I've had a black valve that has been hard to open and close for 2 years on my 29RE. I tried lubing it with no success. While I had the 5th wheel at home last summer, I crawled under the RV and removed (8 screws) the huge sheet (7' x 7') of hard black plastic to expose the underside. I had to then cut the fiberglass insulation to see the valve. Physically, the valve is nearly below the valve handle. The valve is wedged between the outlet of the black tank and a 4" black PVC pipe that goes to the sewer connection.
I started to remove the 4 bolts and nuts that tie the 3 pieces together (pipe, valve, and black tank) and then decided to actuate the valve handle. It moved VERY easily. Surprise.
In looking at the way everything was aligned it was obvious the valve body was being pinched more on one of the sides than the other 3. There was nothing I could move or change to fix this alignment.
My solution was to not tightened the nuts and bolts to their full tightness. I would tighten some and then check the valve movement. Tighten some more and check again. As long as it wasn't leaking and the valve operated more smoothly I was happy. It STILL is not nearly as easy to operate as the two gray valves but it works OK.
Just in case you want to try this, the black plastic piece is very hard to put back into place.
Pat & Karen
2012 CC Silverback 29RE
2013 Ford F250 Lariat, Crew Cab, Short Bed, Diesel, 4WD
Retired and trying to travel where it's warm.