A vehicle such as a terminal or yard tractor used for freight spotting is provided with one or more individual, portable LPG cylinders of the type used by forklifts facilitating tank swapping on an as-needed basis for reduced downtime. Each auxiliary cylinder is a portable, removable and replaceable cylinder having a fuel capacity less than that of the vehicle's primary tank. A controller interfaced to a sensor and a pump is operative to pump fuel from the auxiliary LPG cylinder and into the fuel line to refill the primary LPG tank when the fuel level of the primary tank falls below a predetermined level. The invention may be factory-installed onto the vehicle, or may be provided as a kit enabling an existing vehicle to be retrofitted with the auxiliary LPG cylinder(s).