Faulty City Water Inlet

I have a Realite 27-foot motorhome with a plumbing problem that I have not been able to resolve. When using the water pump while dry camping I vent water overboard through the city water hookup until the pressure bleeds off after the pump switch has been shut off. There is a separate filler tube for the water tank. I have replaced the water pump, which worked fine, but this has not helped. I have looked for a back flow preventer in the line but can't find one. After the pump, the line is teed into a line that carries the water to the sinks and water heater in the rear. The other direction from the tee, within a foot, is another tee where water comes in from the city water hookup. That line terminates at a drain valve. All of this is located under a dinette seat and above the water tank. This problem started three years ago and has gradually worsened. Any ideas would greatly be appreciated.
Don, (Ft. Myers, FL)

Don, it is apparent the check valve (also known as a backflow preventer) built into the city water inlet no longer seats properly. It may be faulty to the point of replacement, or it could simply have some debris caught in the mechanism. An attempt to reseat the seal can be performed by turning on the water pump, removing the plug from the city water entry and sticking the eraser end of a pencil into the city water inlet, depressing the spring assembly. Be aware, you might get a little damp, but you also might remove the debris and allow the seat to seal by pressing in on the spring a few times. The pump pressure may be enough to force any contaminants or debris back out the inlet.

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.