Don, (Ft. Myers, FL)
If the problem persists, it will be necessary to replace the entire city water entry or add another in-line backflow preventer at the outlet of the city water inlet. In other words, the new backflow preventer should be installed at some point just after the city water inlet. It’s usually easiest, however, to attach it directly to the back of the inlet. All city water inlets will have a built-in check valve incorporated into its design. The water pump will also have one built in to the outlet portion of the pump. A third backflow preventer can/should be installed directly to the cold inlet on the water heater to keep the heated water from migrating back into the cold system.
I’ve actually seen a RV that had a cold line tee’d so close to the water heater inlet (and plumbed to the toilet) that whenever the toilet was flushed, hot water (and steam) emitted from the toilet bowl. I won’t comment further.