Also I bought a "Y" water valve so that I have the spare hose to flush with.
I would give that "Y" valve a second thought. Even with the backflow valve mentioned above, you have a hose going to waste water with a direct connection to a fresh water hose going to your unit. If you lose pressure or get negative pressure on the fresh water side it could siphon from the black side very easily. Too much chance of cross contamination from my thinking. I would ensure that any hose you connect to the flush unit is isolated from any fresh water source when not in use and disconnected when done. JMHO.
I understand totally about the backflow preventer and siphon thing however I only hook up my flush hose after the tank is empty and then flush it out. When I am finished I disconnect the hose. It could never siphon back because the spray nozzle is above the liquid level in the tank. I also use the "Y" valve hose for washing the outside of my rv. Good points Army but I am aware and careful.