We had what sounds like this same leak issue. No matter what hose we used, the water would leak at the connection on the trailer. Behind the little round filter is a valve (backflow valve?) that can get stuck. If you take out the little filter, you can push the valve with your finger to loosen it and pop it back into the correct position. Replace the filter and you should be good to go. If this does not work, you may need a new backflow valve. Just a suggestion that worked for us.
Don and Laura + Critters, Eloise and Cooper