Funny how some will say the Flush King doesn't work. I used one on my last TT for 10 years and never had a plug. I also NEVER left the black tank less than about 1/2 full of water.
Both my Mom and Dad (different 5th wheels) have had mounds grow that took hours of work to get rid of and they both have in tank flush systems.I can't say wether they use them right or not but I can say they have had problems.
My brand new 5er has a flush system and the first trip out I used it. Let it run through the flush king till no more chunks came out. I turned it off and refilled the tank with the FK and dumped it hard and got quite a bit more stuff out of the tank. This tells me it is doing something right
Chris