Run it along the frame rail or if you look right infront of the rear axle you will see you have a ton of dead space across the whole body frame to frame. You could run a sch 30 6-8" pipe frame rail to frame rail possibly there as I do not think the drive shaft would come close even sitting on the bump stops. Then run a fill port to one of a few locations. One woudl be to look for a drain plug in the floor pan of the cargo area. I have not looked but I know most have them at various places. You could run hose to that and mount a bulk head fitting there. The other is come off the end and run up to the fuel door and mount a bulk head fitting right next to the fuel port inside the door. Run a small secondary hose as to allow for air displacement and you could fill it fast.