I use a shower shutoff valve on the vent line. These valves work good because they don't shut off 100% and are designed to dribble a little shower water. Then when you're filling the tank if the valve is closed you can hear air escape and it reminds you to open the valve if it was left closed.
My problem was when I go to the sand dunes to dry camp for several days, I fill the tank to the very top at the last opportunity. When I would drive the last 25 miles it would splash/siphon out 1/3 of a tank. Now I arrive with 99.9% full tank.
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