Here is typical trailer water flow path. Of course my POPUP trailer only has the sink but this gives you an idea how the thing works...
The only way I can fill my fresh water tank is pour it in the fresh water inlet myself. Also not this is the only time you should be using the water pump. Its purpose to is pressure up the system when being fed by the fresh water tank. The city water port will pressure up your water system when you are hooked to it...
just my thoughts
Roy Ken