I don't know about tonneaus
Pickup Truck Tailgates | Fuel Economy - Consumer Reports News
I'd say a lawn edge aeromod in the front (maybe down the sides too)
https://www.google.com/search?q=lawn...w=1113&bih=866
Partial grille block... something easy to remove when you tow.
A Scangauge or Ultragauge... that'll probably be your single biggest mileage gainer.