I think the risk of not having a backflow preventer is being overstated.
A couple of years ago I had a question about my black tank flush system and called the company that made it. The owner and designer of the system happened to answer the phone (small company). Among other questions, I happened to ask him about all the failures I was reading about on this forum. He said his model had very few failures, but if mine failed, I should call and he would send me a replacement for free.
He also said that he believed a backflow preventer was completely unnecessary as long as the supply line rose at least 2-feet above the tank before dropping back down to the hose connection. He said there was no way a siphon could form with 2-feet of air above the tank. If mine should fail, he recommended that I just bypass it. (He included a backflow preventer in the kit because RV manufacturers wouldn't buy it otherwise.)
You can take this for whatever you think it's worth, but at least try not to beat up the OP so hard.
