thanks 4 posting pix of the back flow valve, 2dabeach.
1. besides being installed backwards OR incorrectly, it can also fail secondary to debris clogging the flow AT that valve.
2. often IF the valve is the issue, water will FLOOD the trailer inside/behind the lav sink, on to the floor or down the wall.
3. the btf can also become PLUGGED at the perforated head INSIDE the tank wall.
4. even with NORMAL flow, the btf adds water VERY VERY SLOWLY to the black tank, at most a trickle.
i've completely abandoned using it (btf) because of all 4 issues noted above...
its easier to add water to the bt via the toilet foot pedal OR a flush king....
search for the threads on the BTF or FLUSH KING, for lots of related info.
all of the true things that i am about to tell you are shameless lies. l.b.j.
we are here on earth to fart around. don't let anybody tell you any different. k.v.