You are correct...when you are hooked up to city water with an empty fresh water tank, the fresh water tank should not fill up internally from the city water....that is, unless it's been re-plumbed with a water pump bypass to allow filling from the city water connection ( in which case there would be valves to open and close routing the water around the pump)...and if it's new, that's doubtful. It sounds like your water pump (which you do not need to use when you are connected to city water) may be allowing water to back-flow into the fresh water holding tank when it's under city water pressure. It should not be allowing water to flow that direction. If the tank is getting filled to the point that it's completely full, you will have water overflowing on the outside through the overfill tube/hole next to the fresh water fill port....and you did not mention that happening, so it may be a small "seeping" into the fresh water tank. Check in the dinette street side seat where the pump is, to see if anything looks like it is not behaving correctly... and have it checked out...it's still under warranty.
TB & Greg and Abbey Schnauzer
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