fresh water tank overflow syphoning tank dry

So we have a Winnebago Motorhome purchased last year. Worked great all last year and most of this. My normal routine is to fill fresh water tank till it flows out the vent. It will run out for a bit then stop. Drive to campground and all is fine. Now get to campground back into my spot and water is running out the overflow. How can this be? There is no tube in the tank, just a 90 degree fitting off the top of the tank, a 4” piece of hose that goes horizontal to the tank and another 90 degree fitting with a hose going down under the coach. How can siphoning occur without a hose in the tank? Even if siphoning did start once water sloshed to the other side of the tank the vacuum would be broken. Was at the Winnebago factory yesterday and today and guess what. They don’t understand it either. Yesterday they put a loop of tubing in the vent line that went above the tank then down. Gave me the coach back and said all would be good. No way it could siphon with a loop above the tank. Nope! 70 miles down the road stopping at a wayside got out to look and sure enough it is running out the overflow. Turned around and went back. Today the change the angle of the 4” piece of tubing and raised it about an inch. Drove home and it appears to have worked. Will want to test more before any cheering. What I am still questioning is what part the gravity fill port might have to do with this. Never had one before this one. If that lets air in and siphoning starts it could contribute to the problem. If no gravity fill port, no air could get into the system and eventually there would be negative pressure in the tank stopping any siphoning. The way it appears now the tank is somehow pressurized.