You are making a conceptual error related to "terminal velocity". Terminal velocity is that point at which the gravitational acceleration is exactly countered by the drag of air (essentially friction) on the air frame. Since the fuel in the tank is not experiencing the air flowing against it, it continues to experience the gravitational acceleration. It will "feel" 1 G downward force.

Watch the video, and you'll see it in action. Terminal velocity is not free fall. Your assumption that the fuel would not move is correct for free fall in a vacuum, where the object would continue to accelerate, and would not reach a terminal velocity. (for you purists, I know we're not talking about relativistic speeds. Newton is a close approximation for what we're dealing with).

Brad