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Did you buy a pressure limiter for your water hose?  It's a gizmo that screws onto the end of your hose and then you attach that to your trailer.  It reduces the water pressure so you won't blow any of the trailer's plastic water lines.  A hardware store or RV parts source (such as your RV dealer) should have them.

It kind of sounds like the water is getting forced backward, backing up into the fresh water tank.  I am guessing that KZ would put a one-way valve in the water line between that tank and where it goes into the rest of the water lines.  Perhaps the city water's pressure was too high and was forcing water the wrong way through that valve.  Or the valve was faulty to start with, or it simply failed.

I don't claim to have any direct knowledge about such a valve… it just seems to my reasoning to be likely… perhaps someone else knows more and can confirm or correct my supposition.