[Motor31]

I think you are each talking the same thing, just differing on semantics. By it's design, a check valve IS a back flow preventer as it will only allow flow to go in one way.

The valve in behind the shower is also a vacuum breaker style back flow preventer. This just stops the siphon action from the black tank rather than having a physical flow preventer like the check valve.

In each case reverse flow has been prevented. In the water heater it has to stop water under pressure from flowing backwards. In the black tank flush case it prevents siphoning rather than pressurized flow. If your black tank is under pressure, lay off of the beans!