I still remember the Mythbusters Episode where they showed the best MPG was by leaving the bed the way god intended, and by that I mean no bed cover gate up. Something about it creating an air pocket in your bed for the air to ride over. Now they did say that was only true if you were driving over a certain MPH, I believe 40. Otherwise the air pocket wasn't created, not sure what was best in conditions under 40.
I know the new ram '13 all its active grille shutters and bed covers and the extended step, they were advertising that the individual increases were under 1%. So really none of it is really going to make any difference.
What would interest me is with Diesel trucks, they have the same stock tire size, but with the torque output I'd wonder if it wouldn't move a bigger tire around efficiently enough to actually have fuel savings.
You could lower your truck, that would give it a lower surface area, you'd need to keep the stance stock and stock wheel/tire size, that would make you more aerodynamic possibly, but if the stance was changed you could end up being less aerodynamic.
But yeah all in all like everyone else said, you're talking about 1-2 mpg probably.
