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[Judge]

I stopped being lazy and pulled this out of the 2019 Ford F-550 Manual:


Dual Fuel Tanks (If Equipped)

Your vehicle may have a dual fuel tank
delivery system which operates
independently and automatically. The fuel
pump in the front fuel tank delivers fuel to
the engine. When the fuel level in the front
fuel tank drops below three quarter full,
fuel automatically transfers from the rear
fuel tank to the front fuel tank. Whenever
there is fuel in the rear fuel tank, the front
fuel tank level remains between half and
three quarter full.

Note: If your vehicle runs out of fuel, you
must add fuel to the front fuel tank to
restart the engine.


So we can say for certainty that the front take is used first (assuming it is full)and until it reaches 3/4 full and then fuel transfers from the rear tank to the front tank. As longs as there is fuel in the rear tank, the front tank will remain at ~3/4 full.