I KNOW for a fact there is a backflow preventer on the inlet of my black tank flush. I had to replace it once. It cracked and flooded the bathroom. The way it is plumbed the connection goes through the wall into the backflow preventer then down the inside wall under the sink through the floor into the black tank. I see no way for anything (germs, feces or the like) to get from the black tank, three feet straight up the pipe to the backflow preventer, through said preventer then outside to the hose connection. When I have full hookups I unhook my freshwater hose then hook it to the flush and when done flushing hook it back to the city water inlet. No problems.
