Other easy areas to look at:
-Mirrors
-Fluids
-Driving techniques
I think the added weight from the tires will cancel out any gains from the lower rpm's unless you are doing drastically more highway than rural/city driving.
Smaller mirrors or adding aero forms to the fronts of the existing ones can be a good idea.
I've experienced anecdotal gains of 2mpg switching to synthetic fluids all the way around. Then biggest gains are supposed to be in the differential.
With a truck driving habits are going to go a long way. Any time you have the chance to sacrifice some speed, let's say by going 55 instead of 65 you are going to notice huge results. The worse your aero is (and yours is terrible at best) the more higher speeds kill fuel economy. Here is an example of how Diesel_Dave drives to get 328% above EPA with optimal driving (along with some mods).
http://ecomodder.com/forum/showthrea...a-26259-5.html