Why is fresh water tank filling up when hooked up to city water?

There will be a valve somewhere along the water line coming in from the hose hook up on the outside. You open this valve to fill your tank, watching your tank fill gauge so you can turn it off once full.

James, Another possibility is the "backflow" preventor that should be on the output side of your pump. It is designed to allow water to go from your tank to your plumbing but should not allow city water to flow backward through the pump to the tank. Depending on your model it may be an inexpensive plastic part between your piping and the pump. Occasionally a small piece of debris, usually from manufacturing the tank or winterizing, can keep the valve open .Remove all pressure, remove the preventor valve and blow through it. You should be able to blow through in only one direction.

Many RVs have the plumbing to fill the fresh water tank from the city water hook up. This valve is usually near the city water hook up and labeling is similar to "city water" and "tank fill". Another anomaly from leaving the valve in the "tank fill" position is that, when not connected to city water, the on board pump may run continuously and the water volume is very low.