I guess that's why you didn't see the water pump!
On my trailer, the FW fill is at the rear of the trailer while the city water fill is on the left side. On my previous trailer, I couldn't find the water pump either so I would turn it on for a few seconds at a time and then try to locate the noise. I found it after a few tries directly behind the electrical panel, which I had to remove to gain access to the pump.
Talk about the wrong place to locate a water pump!!!
Maybe someone with a model like yours will chime in with a location. It will probably be located behind a panel you'll have to remove to gain access. I guess you checked all around the sink area? What is directly inside your trailer from the FW fill?