Suffice to say a backflow preventer is required by code. If it wasn't, FR wouldn't install it. There is a direct connection between the black tank and the campground's potable water supply system so the water supply system has to be protected from contamination.
I use an external brass one like the picture B & B posted because the original owner of my TT removed the factory version.
2011 Rockwood Signature Ultra Lite 8293SS, 12K Equal-i-zer
2016 Ford F-250 XLT SuperCrew, 6.2L, 4x4, 6'9" bed, 3.73
2010 Rockwood Roo 23SS (2012 - 2014)
2009 Ford F-150 XLT SuperCrew, 5.4L, 4x4, 5'6" bed (2012 - 2016)