When we bought our used 2010 Four Wind I was checking things out and found after the first dump some paper looking material at the end of the pipe. I just washed it off with the hose but I noticed some glue build up like a spike. I guess an excessive amount of glue on assembly, with the pipe up side down formed this spike. Our black tank is on the passenger side of the MH and the grey is on the dump side so there is several fittings and couplers before the dump valve. I was able to break the spike off and will have to watch at each dump for additional problems. After dumping we add water and treatment so it sloshes around as I drive. I just wonder about the connections I can't see, I tried shining a flashlight up the pipe but can only see to the first elbow. It seems to drain ok so I hope that was the only road block!!!!!!!!
Go safely.