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.