Another bad crossover valve.

[Chamberpains]

Fuel tanks are always drawing air from the outside thru that breather valve when the truck is running. It's a very slight suction. Its occurs because of the fuel being sucked out of the tank. That breather has 3 purposes. 1 is to keep the tank pressure from going in to a vacuum because of the constant fuel being drawn from it. The 2nd is to keep the fuel from sloshing out or dumping out in case of a roll over and 3rd is to vent pressure as the fuel heats and expands in the tank. Thats why its kind of a one way weighted sliding ball valve of some sort in all of them. The ball adds a little weight to keep the valve open to vent pressure. Without it the vent would simply push shut under a pressure building situation. Since there is no filter on the breather to keep dirt from going in the valve/breather, you sometimes get grit and grime in them and when you hit a hard bump or dip the valve bounces and sticks shut. Causing the tank to go in to a vacuum which then makes it hard to pulll fuel from so it defaults to the path of least resistance and draws off the other tank.
Mack's don't use breathers on the tank instead they use an integrated fuel cap with the breather in it. Switch your fuel caps to those and I'd bet your problem goes away. Just make sure to clean and rattle the cap every now and again.

Also that long hose coming from the breather is important to keeping dirt and dust out of the breather longer. That's just because the length of the tube is hard for dust to travel that far without getting stuck to the tube walls. Then fuel vapor coming out washes the dust away over time.