Options for black tank flushing?

I question the check valves on these things that are to keep from contaminating the water supply. You take a $20 device and it has a built in check valve of some sort. You then connect your poop tank to a drinking water supply and expect there not to be any contamination. Meanwhile, go to any Walmart or hotel and take a look at the massive multi-thousand dollar backflow preventer between the sprinkler system and the public water supply that also must be tested every so often. And this is to keep stagnant, rusty water from contaminating the public water supply, not poop water.