By passed Black Tank Flush back flow preventer

We don't know how far the OP's lash-up is above the waste tank.

What he obviously does not know is that a real backflow preventer is more than just a check valve. When pressure is not present in the inlet or is at the outlet (either case) the device opens a 2" air gap between the supply and the service.

But it will be safe for the community as long as he never attaches the system to the water supply and/or overfills the black tank.

Matt