Does your trailer have separate fill openings for city water and fresh tank water? Or do you have one hookup and you flip a lever to direct the water?
If two fill openings the city water line most likely comes into the trailer behind a cabinet or under a piece of furniture. Have somebody stand by the water hookup and go inside the trailer and see what is on the inside wall. You'll probably be able to take out a drawer or look under or inside a cabinet and the fitting will be there.
Follow the line from the outside wall and cut out the first T that you come across. If the T is not clogged then put a bucket at the end of the hose inside and have someone turn on the city water for a second or two. I'll bet you find the problem is right there at the first T.
