The fresh water tank in my Scamp
has a small hole, ~2 in., on the top, with a cover. That cover was not water tight and when towing with some water in the tank the water sloshed enough to get through. Once I found some water on top of the tank, as well as on the floor around it. Filling the tank to the point of overflowing also led to having water leak through the top. Take a good look at that area, that might be your problem.
I sealed that little cover with Lexel and have had no problems since.
Some people assume that it must be condensation, but I am very skeptical about that. None of those recent posts mentioned filling with ice cold water in really humid weather/climate. In my Scamp
the tank is enclosed, under the seat, with very little or no air circulation, so very little water should condense on the tank in any case and should not lead to a soaked carpet.