The tanks do have a vent so the fuel will flow by gravity naturally. I suspect that vent connection is more than just a right angle barbed fitting. I pretty sure there is a check valve in that fitting. It might be spring loaded to hold the valve open so the fuel will flow and then when the tank gets full (if you fill the tank to the neck), the valve closes to prevent fuel flow from the vent hose.
The only complaints I've seen are some say the filler hose check valve doesn't seal adequately allowing fuel run out under the cap and down the side of the truck. They also go on to say it happens when they are four wheeling or riding hard on rough roads.
We don't have that problem while pulling the money machine back there.
My tank vent hose just drops through the bed.
2013 Voltage 3800, 2012 Chevy 3500 HD
2010 Yamaha V Star 950
2009 Yamaha Raider
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