No real "rocket science" here. In general, the lower the placement of the direct fill line(s) on the rear of a tanker the easier it is for the fill site crew to make and break connections. This generally translates to less time spent at the fill site and a better overall Tanker Delivery Rate. Some manufacturers are better than others at fill line placement, so unless you are buying some stock vehicle, take time to get the connections where you want them and make sure the connections are easy for one person to access and operate. It is pretty hard to make any changes once you take possession of the rig.