Since we have owned this trailer, the black tank flush has not worked. I've tried everything possible to fix it. Replaced the back-flow valve, filled the tank up several times with different additives, letting it sit for weeks full with additives, blowing it out with air pressure and adding bags of ice cubes after emptying and then travel to next site. Natta! nothing worked. I've installed one of those wands that has a shut off on it. I was able to cut a hole for the hose inside the refrigerator compartment wall. ran the hose from there to the wand so all I have to do is open the outside ref. door and connect the hose. I then made a bushing to go inside the head to keep the valve open and it also serves to keep the water from splashing back out by the inside hole of the bushing being just big enough for the wand to go in.