Tex - Thanks, I may have assumed too much. Below is the "air break" I installed on my MH to stop the fresh water from siphoning out.
1) The gray pipe is the tank overflow line, and originally it was one piece that ran from the top of the tank and exited under the coach. This arrangement allowed the water to be siphoned from the tank whenever the tank was filled to overflowing.
2) I cut the gray pipe and connected it to an elbow and a short piece (3") of the pipe I cut off.
3) The portion of the original gray pipe that went under the coach was replaced with a section of PVC piping (The inside diameter of the PVC must be bigger than the outside diameter of the gray overflow pipe.). It was easy to install, since it fit through the hole already in the flooring for the original piping.
4) Position the PVC pipe so it's as high above the top of the tank as possible, and insert the small section of gray pipe connected to the elbow. The connection should be loose, so secure the lines so it won't pop out.
This "air break" arrangement will provide overflow protection while making a siphon impossible.
Good luck. - Dave