RV MFG.s are 'required' to install those anti-siphon vacuum breaker devices on black tank flush system due to POSSIBLE cross contamination of waste water with potable water
Possible but NOT very PROBABLE
Possible because potable water source is connected to a system that feeds into a waste water system
NOT PROBABLE due to flush system design and the plain old fact that multiple errors would have to happen (the perfect storm ----possible but highly unlikely)
Black tank would have to get FULL....so full that flush nozzle at top of tank is under water-----and then level would have to go up vent pipe far enough to be above the top of anti-siphon vacuum breaker which are high up under bathroom sink cabinets.
Then it would have to flow out hose connection which may or may not have a plunger check valve (like city water connection) and then flow thru potable water supply hose and into water source system which would have to be completely depressurized for water to back flow from RV into Potable Water System.
Possible............just not Probable
Those anti-siphon vacuum breaker devices are OPEN to atmosphere (top cap covers opening) and when internal plug doesn't pop up and seal opening off water sprays into bathroom cabinet----great!
Remove device----replace with couple of 1/2" PVC elbows
Change out flush system hose connection with one that has a check valve (City water inlet type) if yours doesn't have a check valve already
Use a hose end anti-siphon vacuum breaker device like this one
No more water spraying inside RV
No back flow issues
BUT one should use a pressure regulator at water spigot with a 'wye' hose connector.....one side for potable water supply and other side for flush system
Water pressure regulator on flush system..........high pressure can damage spray nozzle. It is just plastic with holes drilled into it.
Besides 50# regulated water pressure is more than adequate
Dump when FULL ---high liquid level/volume/weight will carry majority of stuff down and out. Quick flush/rinse is all that is needed to spray down sides and clear bottom of tank.
They are holding tanks and don't need to be spic-n-span clean.
Add a few gallons back in after dumping ----enough to fill dump line and keep bottom of tank wet