It would help to know what model you have, but I will give you an answer that usually covers the trailers that have a gravity fill water tank and the city water hook up is just used to supply water to the trailer kitchen and bathroom facilities.
All you have to do is make sure the pump is turned off. The pump has a built-in check valve that does not allow water to flow back into the tank. That is all I had to do when I owned ny trailers.
You can put the information of your trailer and tow vehicle in you signature in your profile that way it will show up on your every post and won't have to keep typing it, like mine does.