How would you ever know? Wind tunnel test? I'm no authority on this topic but I think it would be negligible and maybe not even measurable. I guess you could try checking mpg with it open compared to closed. There are a lot of other things that would impact MPG rather than your sliding bed cover.
The wing, however might help especially if your 5th wheel cap is rather flat. Mine is very rounded and curved and I doubt if a wing would reduce wind resistance very much.
I think slowing down to 55mph would have the greatest impact on fuel consumption.