Is the SaniFlush back flow purge valve really necessary.

Unless you want to drink sewage water, use the back flow preventer. To be honest, they should be installed permanently on every campground faucet.

Back flow diverters are required to protect everyone else… yes it should be replaced…

CLARIFICATION: I am talking about the SanTFlush on the back end of my TT. It is the tank flushing winterizing purge 'button' inside the SanTFlush inlet for flushing out the black tank. The winterizing instructions advised that one should push in the purge button as part of the process to allow any water still in there and under pressure to come out before it freezes. There must have been so much pressure that the entire guts sprang out of the fixed hose inlet fitting that is mounted to the outside rear wall.

In order to replace it I would need crack the seal around that fitting gasket, remove 3 screws, and somehow unscrew the fitting from inside the wall (the replacement appears to screw into something deep in the wall and then seal and screw the attached flange back on the wall). My SanTFlush that is missing the purge button is so well fixed in place that I hesitate to monkey with taking it out if really not all that necessary (even though I have the replacement part). It just seems superfluous if all it does is prevent black water from backing up into the hose if you forget to watch over the SanTFlush process when doing it (which I don't intend to let happen).

You can get a portable backflow preventer that attaches at the hose fitting.

Back flow is unlikely but considering the results if it did happen, I would replace the valve. They are there for a reason.