When I put fuel quality I was meaning by brand, if you run premium with the truck tuned for 87 the majority of the fuel properties are not being taken advantage of. Raising the timing a few degrees would help. As the price of gas has gone up I have switched over from the cheapest fuel to brands like Shell. Already my mileage is improving along with overall performance like a smoother idle. When I see a 5 cent difference I really see (.05/3.xx) or less than 2% cost increase. The switch to higher grades will probably be coming because it will be more cost effective to use them as the % price difference narrows.
I dont think they make the wheel skirts for these trucks because it would not be very "popular" and the wheel covers may help if you find some that still have openings because like you mentioned the brakes need to be easy to cool.
Air dam under the bumper
I was thinking you were talking about a grille cover. I'm sure the skirt would help.
I figure those rally cars see more gravel, mud and dirt in one race than you do in a month. If you leave openings it should help some of it come back out. It also doesn't have to be a full covering. You could stop at the tranny.
My AC doesn't do much at idle, guess I need to recharge it. But that is part of UDPs. They lower the gear ratios for the accessories.
For gears, do not go below 3.55 if you want to keep any useful towing capacity.
You should look into lowering shackles in the back. It will not make the truck useless because you can still tow somewhat lighter loads with them. If you still need to tow heavy trailers you can look into helper springs, air bags, etc. I've seen them put on trucks and it brings the back in line with the front. It also lowers your tailgate a few inches.
Overall, your driving style has much more say in mileage than mods. If you are on the highway, drop it down to 65 rather than 70. If you are in the city, both Jack and I have found that giving it more gas to get up to speed then backing off to maintain improves mileage more than granny driving it with turtle slow acceleration.