G'day Brad,
If the plane reaches terminal velocity, so does the fuel in the tank, so it stays where it is, for the fuel to move forward, it must be moving faster than the plane, how can it be moving faster? Gravity can't do it, because the plane & everything in it, is moving faster than terminal velocity.
The only way that any of this can happen, is if the plane is slowed down, by possibly a large prop, acting as a break, but that is unlikely, as most pilots don't throttle back enough to cause this slow down effect, they do full power dives, usually the plane pays the price, by snapping the wings, when it is pulled out of the dive.