On my trailer the leak was under the sink. All it was a loose conection on the cold water to the sink. Our trailer also has 2 drain valves by the the dump valves. I also have a pressure regulator on the trailer and on the hose. I know it is a over kill but I have been to camp ground with a lot of pressure.
My advice is hook up the hose and turn it on and start looking .
The rubber hose should not be there. In my trailer it is all plastic connections.
I hope this will help.