Black Tank Vacuum Breaker

From the picture, I would say the black wire tie is putting the Vac-breaker in a bind. It does not take much to jam plastic valves (Over a period of time). I would replace the tie on just one line and see if you can tie the other in a way not to put the valve in a bind.

Now to fix the valve. (With water OFF) Remove the valve, the fittings should be very close to finger tight. If not, that could be part of the problem. When you get it out. Carefully open the check valves (should be easy to tell what is stuck). Lube with plumber's grease ONLY. If it frees up and seems that it should be OK.(If not replace) Reinstall, FINGER TIGHT and leak check.

If you still can't get flush water into the black tank. It's time to attack the check valve where you attach the hose to flush system. That is where I would have started. This check valve normally fails OPEN but you never know. (Make sure you don't have a plugged screen washer, failed pressure regulator or something like that) I also hope your not using your drinking water hose for all this.

Let us know what you find, please.

Yes that looks like it is it located in a vanity cabinet? Run the tank flush and watch it!

We are in a 2001 Winnebago Adventurer... our black tank flush vacuum breaker is behind the mirror that hangs on the wall by the dinette.

We took the mirror off the wall and found an access hole. That is also he access hole to get to the hot/cold water knobs for the tub/shower.

Our black tank sprayer stopped working. We found the back flush valve (behind that mirror) was completely clogged with sand... the black tank flush nozzle inside the black tank is still hopelessly clogged with sand, too, I guess. Word to the wise... It's not just the drinking water that needs to be filtered. ALL the water the comes into your rig should be filtered. At least use a micro screen sediment filter.

That is a fresh water Vacuum breaker. If it went bad no fresh water would go in the bowl
BY Flush your black tank what do you mean.
Toilet won't flush
Cannot empty black tank
Your Black water tank flush system does not work.