The over flow "vent" really cannot be used to "fill" the fresh water tank. Why? Because if you attempt to insert anything into that very small-in-diameter tube you will create a vacuum. There's nowhere for air to escape and you'll never "fill" the tank that way. Some air may escape out same tube, but it will probably come shooting out under pressure first.
As rohrmann said above, flip the valve to "fill" then flip it back when done.
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2019 Montana High Country 375FL
2014 Chevy Silverado Duramax, 6.6L Dually
