The AC unit could only hold a few ozs. of water and than only if the trailer was tilted away from the hose. That amount of water would evaporate in weeks from the last time it was used.
I agree you have to look at the freshwater holding tank. If the hose in question is on the sane side of the trailer as the fill port that strengthens our suspicions.
Tilt the trailer to the side the hose is on. That should drain the tank in short order. If after that the water reappears that would lead me to believe there is a leak in the trailer and water is making its way to the top of the fresh water tank and collecting in the tank. I say this because some trailer tanks are mounted with a flange at the top and under extreme cold the tank shrinks enough to crack the top of the tank. The tank can be dropped and repaired. When remounting oversize the mounting holes so the tank can float on the mounting screws. Been there had to do that.
WBCCI 12156 AIR 3144 WACHUNG TAC NJ6
2004 Excursion 4x4
1991 34 ft. Excella +220,000 miles, new laminated flooring, new upholstery, new 3200 lbs axles