Assuming you have a "gravity fill port" for your fresh water tank, there are two hoses attached to that fill port. One is the 1.25" fill tube, the other is the 3/8-1/2" vent tube. Supposedly, as you fill the tank, air is displaced out the vent tube. When the tank is full, water is displaced out both the fill tube AND the vent tube. If either is not securely fastened AT BOTH ENDS then water can leak into the trailer from either tube.
Which is your "problem connection" ??? To know that, a visual inspection would be necessary.
Keep in mind that you may also have a leak from a split in one of those tubes, from a split or crack at the tank, a leak in the city water connection that only leaks under "some/not all" conditions or any number of other issues...
The only way to know for sure what is leaking would be to actually inspect it.
2015 F250 6.7l 4x4
2014 Cougar X Lite 27RKS