Just used the Flush King for the first time last week.
This was our fourth trip in the trailer. We thought we were doing a good job getting rid of the waste before we left. WRONG!!!!
Had the DW monitor the inside while I filled the black tank. Took about 4 fills to get the water running clear. Used it again to fill the tank about 1/3 full and added a dish soap packet and stinky packet.
Get a different hose as recommend.
Next trip I will use the icecube suggestion to see what it does.
I would recommend this to you as I had several other campers come over and watch. After the results they said they were impressed. Can you believe there would be adults standing around watching a black tank empty.......
Here is a picture. It has a backflow valve in it but I used another one as well.