I am going to add my two cents worth so maybe my experiences can help someone. I too had problems with the black tank not wanting to dump when I knew it was full. This started about two months ago. Please note that I am full-time and have lived in the RW for about one and one-half years. I typically use the black tank flush after each dump to clean the tank out. I assumed it was a broken or inoperable cable on the dump valve so I went and purchased another one. I waited a few days as I was not looking forward to doing this job. Before I pulled the underbelly down I tried dumping one more time and it appeared to dump OK
(I have a clear elbow going into the septic tank inlet).
Fast forward two months and the same problem occurred. I partially pulled the belly pan down and confirmed that nothing was wrong with the tank valve. This led me to believe it was a plugged valve or tank opening. I purchased one of those back flush devices with the clear elbow, water hose connection, and gate valve. Well I closed the black flush valve, opened the black tank valve, and then turned on the water. I did not run it long as I knew the black tank was almost full. I turned the water off and opened the back flush valve and started to get waste passing through. I did this several times and appeared to keep getting waste passing. After completely draining (at least I thought so) the black tank I went in and poured several cups of laundry detergent down the toilet and went back and started back flushing more. I figured I would sit down a minute and enjoy a cold one for my victory (water only for this boy). I know only 3 to 4 minutes transpired when I heard a loud noise and water was dumping from the underbelly. And it was not totally clean water either
! It appears there was still waste in the tank and it did not take as long as I thought to completely fill the tank up.
I lowered the rest of the belly pan as I had a literal mess on my hands now. On my RW the black water drain pipe was only inserted into the black tank with no adhesive. There was a pipe clamp over the conduit but for what reason is beyond me. Looking back I am glad it was not glued as it allowed the pipe to partially move back from the tank when I over pressured the tank. I took a long break and let what water was in there to drain out ( the pipe did not complexly come out). I then reached underneath the drain pipe and pushed it up and it popped back into the black tank opening.
I must mention that I would occasionally get a sewer smell underneath the RW. I did not know if it was from the coach or my septic tank opening (I keep it capped when not dumping). I now think that I may have been getting some smell from the pipe tank connection since it was not glued. The foil barrier showed some evidence of intermittent moisture from somewhere. I am going to add a mastic material we using on medium voltage electrical connections to prevent moisture ingress. I am going to place it around this connection. In addition, I am going to place a 3 inch plumbing repair doodad made out of rubber with two hose clamps. I am going to have to shorten it and also cut a slit in it to place it around the pipe. I really think the mastic alone is sufficient to stop possible water seepage as it should not occur if I do not add excessive pressure to the system.
Make sure you do not go skimpy on the water usage using the black tank as a full timer. It can cause the waste to solidify, partially or fully blocking the valve and/or tank opening.
If using one of the clear black flush elbow valves be very cautious and never walk away from it. Run it awhile and then dump until clear.
In between using the back flush elbow also use the RW back flush as it seems to spray the upper walls in the tank and assists in the cleaning process.
I have been flushing every once in a while for several days now and almost got the tank perfectly clear now.