When you say that the cab and box flex, you mean they move independently of each other? The cab could be going one way while the box goes another way? Aren't they both mounted on the same rigid frame, or is one or both of them on some kind of floating mount? To get around that, I suppose the hose from the filler to the tank would have to also be flexible or mounted in such a way that it could move. You certainly wouldn't want the tube to crimp or break between the cab and the box next time you hit a pothole.
