New to my 2017 Four Winds 26B and came across a real stumper. Trying to drain the black water tank and nothing came out! Figured it was plugged with something but not a clue what since we only use the toilet for peeing. I bought a flushing valve and it would build up so much pressure that it would blow it off the connection. I finally looked under the off side of the RV and lo and behold, another gate valve for the black water tank. I have always pulled tow behinds before this class C and have never seen such a configuration before. Live and learn and we are learning quite a bit with this RV. The walk through never covered this valve.
Our 24F has the same layout for Black tank drain valves. The valve closest to the tank keeps solids from dropping into the drain line and blocking it. Think of 6-8' of solids in a 3" pipe; can you say "blockage". This is how it was explained to us.
My Axis has the same “extra” valve. My first emptying of the black tank was interesting.
Stay at a park that brought a portable dump station to the site. The operator and I were both puzzled why the tank would not drain. Then we found the second valve on the opposite side of the rig. Problem solved.
Thor Axis 27.7 towing a hot little Chevy Sonic