Sounds like the freshwater tank overfill tube. When I filled my tank I would wait until the water came out of this tube, as I knew the tank was now filled. The water from the overflow tube came out for about 3-5 minutes then stopped.
Bill & Cindy McKenna
2015 Palomino Puma 32DBKS
2005 Ford Excursion 6 liter Turbo Diesel