I have had the same problem with the fresh water tank loss of water and it was very frustrating, especially when you dry camp.
I called Jayco two week ago and talked with Motorized Customer Service Specialist Jacob Frey. He was aware of this problem and sent me an email with two photographs and instructions regarding how to fix this problem.
The three hoses people have described are all vent lines that are attached to the top of the fresh water tank. A quick removal of the panel in the right rear of the coach will reveal the tank and hoses. The pictures and instructions say to cut one of the hoses approx. four inches from where it is connected to the tank. Insure that the cut line is positioned upward and if possible, fastened to the ceiling above the tank, duck tape works well to do this. The loss of water is caused by a siphoning effect. By cutting the one hose vents the tank and stops the loss of water. Remember, the cut end of the hose must be positioned above the top of the tank. Contact Jacob Frey at 800 517 9137 opt1/opt2 or email at
jacob_frey@jaco.com. and I am sure he will send you the email he sent me. I would attach his email to this reply but I don't know how, sorry. This fix does work.
The second thing I did was cut a window in the panel that I had to remove to get to the tank. This window is approx. one inch wide and ten or so inches tall. Now I can easily check the water level. My factory installed led lights that are supposed to do this are a joke. All this took me about an hour, know that I am old and slow. Sam