My father did something like this, but used an electric pump on an in cab switch from the bed tank to the truck tank (feeding into fill neck vent tube). Worked great till he forgot to turn it off when he went into walmart. Pumped diesel on the parking lot. He overfilled the truck tank and it ran out of the tank vent.

A lesson: pipe your truck tank vent back into the aux tank, then vent that tank to atmosphere. When the overfill happens, it circulates into the aux tank, and never overflows.

I would never have a gravity feed directly to the truck tank. It is not DOT legal. If the hose came undone.... there goes 100's of dollars of fuel and you are liable for cleanup costs for the spill. My father paid $6500 after insurance.