I have an 08 and the overflow vent tube is located on the right side of the fresh water tank. I have filled it to the tube connection on the tank before, but I have never filled it until it ran out of the tube. Hope this helps.
2008 Forest River Berkshire 390QS
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Sometimes 6X12 enclosed trailer with a
2007 1200c HD Sportster & a 2006
HD Night Train