I always scan thru the aux tank questions just to get a kick out of the urban legend and totally wrong information on gas aux tanks.
They just have to be plumbed correct and they also can be plumbed into the gas filler neck to replenish the gas in the vehicle tank.
no gravity transfer. Install a fuel pump to pump it out of the dip tube in the aux and into the filler tube of the vehicle tank. Also install a fuel shut off between the fuel pump and the filler neck to prevent siphoning.
One switch turns on the fuel pump and opens the shut off valve. while you are at it install a fuel gauge for the aux.