Consider this simple diagram of the water flow in a typical RV trailer...
For the water from the site city water spigot connection to get to the fresh water tank it would have to go through the trailer water pump as well as the ONE-WAY valve between the 12vdc water pump and the fresh water tank.
I have never experienced this with my setup but have had water come out of the city water connection one time when I was running the 12VDC water pump. I smacked the one-way valve shown on the simple diagram between the 12VDC pump and the city water connection and it stopped.
The only method I have ever used to get water in the fresh water tank on my 2008 OFF-ROAD POPUP trailer is to pour it in myself at the fresh water fill spout.
I have seen some folks mod the system to include a bypass using tee's and another valve to allow the city water connection to fill the fresh water tank. I don't think I would ever do this mod as the pressure of the city water may exceed the over fill port and crack the fresh water tank by having too much water pressure in it. That was an actual recent report on one of these forum.