fresh water tank overflow syphoning tank dry

Let me start by stating that my overflow in a tube sticking out the bottom of the underbelly of the trailer. I have always just filled my tank untill water starts to flow out of the tube. It would always drain off the little bit that got in before I could pull the hose. For some odd reason this year when i do this the water will sit there and keep running out and will syphon the tank nearly dry before it stops running. Anyone have any idea what is going on. I'm thinking about installing a valve on the end but I fear that I would forget to open it and make even more problems