Overflow on fresh water tank

Every time I fill the fresh water tank, it seems to flow out a 1/2" tube near the back of my rig for a long time. Last fill was done very slow so I could stop it just as it was full. I seem to get to the camp area with a lot less fresh water than I started with. Can I put a fitting on it and cap it till I get where I'm going? Thanks for the advice

Your air vent is plugged with water, mine also does that. Try blowing air in the vent to clear the water out.

Thanks I'll try that when I fill next. I thought maybe the water was flowing out of the vent.

I put valves on my overflow pipes. Now all I have to do is to remember to open and close them at the proper times.