The backflow valve itself is from B&B Molders, #571 I believe. Mine is located above the heater and access is easy through the vent panel. Although it makes me nervous to use it, I haven't had a problem in 3 years [yet]. I do carry a spare that I purchased off Amazon for about $35. I can actually hear the water going into the black tank, so if the sound changes, I'll turn the water off immediately. I also use a 40lb pressure regulator on the flush hose.
2012 Eagle 320 RLDS
2012 Ford F-250 Crew Cab FX4, 6.7l PSD
3.55 E-locker, Timbrens