You only need a back flow preventer on the black tank flush. The idea is that any water from the black tank will not be able to get into the potable water source or your hose in the event of failure. That's an extremely unlikely scenario anyway, but an ounce of prevention as they say. The only reason I put one on the black tank flush is to keep all the water from the line up to that high point where the plastic check valve was located from running back into the convenience center when I disconnected the hose. Since that point is higher than the convenience center connection, gravity will bring the water back.
Eventually, I'm going to open the basement wall and locate the white PEX underneath an eliminate the loop that goes up under the bathroom sink and back down to the black tank. No need for that loop anymore.
Sorry for the delay in responding, bigred. I just noticed your message.
I put the backflow on the inlet for the black tank flush, then put a quick connect on that so I can hook the hose up when I need to flush. If that's what you are talking about, then yes, that will work.