3611 RL black tank doesn't drain properly

[wm5e]

Since new our 2015 3611 has never properly drained the contents of the black water tank. Almost every time we open the black water drain it just sits there with nothing coming out. We have to turn the flush hose on and let enough pressure build up to finally push the plug out and start the draining of the tank.

Possibly a related issue is that once the black water has drained and the flush hose run until the water coming out is clear, we close the black valve and then open the gray valve and black tank waste come out. This is frustrating if I just want to dump gray water into a gray water vessel but sewage comes out first.
Has anyone else experienced this and if so how did you handle it? We've had our unit back to the dealers with no resolution.
Thank you,
Gene

[DQDick]

Not sure what is causing your draining issue, but on a couple of occasions folks have found the round floor cutout from installing the toilet was in their black tank. Getting a plumber with a camera on a snake is probably the best way to see what's going on in there. As far as the black tank waste coming out with the grey that's not really a worry. What's happening is that when you open the black tank the flow goes both ways. The main flow goes out the dump connection, but some backs up against the grey tank drain opening. When you drain the grey tank the first thing that comes out is that bit that backed up against the grey tank valve. Try shutting the gray tank and dumping it by its self. You won't see any black tank stuff.

[bigskyjimmy]

Every time I get something blocking the back tank valve what I do is make sure you have a end termination valve then close it and open the Black tank valve and make sure you have a pretty full galley tank and the main grey tank pretty full then Pull both grey tank valves open for a about a minute then CLOSE the grey tanks and then open the End Valve and what that does is backflushes the Black tank and loosen any Obstruction stopping it from draining ,I do that every time I empty and flush the black tank and it works better than any Flush King etc.. that I have tried