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I don't currently own a truck, but I used to own an F150. Hopefully some of my suggestions help.
With the F150 (a 90's model) one of the bigger losses seemed to be gearing - the engine was at very low load in top gear, presumably because it was intended for towing and not as a commuter. As such, it didn't respond well to weight or aerodynamic changes. A significantly taller rear ratio would probably have been the single largest fuel economy improvement I could have made.
However, you asked about aero, so here's my list. Not all of these are practical, but that's pretty individual and subjective, so take from it what works for you:
-Grille block in winter, adjusted to conditions. I expect it doesn't get terribly cold in LA, but a cold engine is an inefficient engine, and cold air blowing over the block extends warmup time. I can reduce full warmup time in my car from an hour to just 5 minutes with a grille block, but I have to adjust just how much is blocked as the season progresses. In addition to the warmup benefits, this mod also improves Cd. OEMs have these in modern vehicles in the form of louvers or vents that open and close as more or less cooling is needed.
-Front air dam - many trucks come with one from the factory now. Ideally it's as low as the lowest hanging component under the truck. I believe ideally it would be tapered slightly to divert air around the edges of the truck.
-Aero cargo cover - best is a "kamm" style with a curved, tapering roofline. Almost is good is a small spoiler (a flat one to give a clean edge, not one that curves upward) along the trailing edge of the cab, with a flat cover on the bed. A standard topper usually increases drag, rather than decreases it.
-Side skirts can help. I imagine the best height for these would be the same as the air dam.
-External mirror reduction or delete. I sometimes swap the mirrors on my car for small cameras, which have virtually no frontal area, and no blind spots like the mirrors have. Some people choose to run smaller mirrors.
-Smooth underbody paneling - this can be combined with an air dam or replace it. Any smooth sections help, it isn't all or nothing. I find I can get away with cheap corrugated plastic from big box stores (Lowe's and Home Depot sell them in 4x8 sheets) as long as it's away from the exhaust.
-Lowering - even a slight drop can help reduce drag.
-Rear wheel skirts - a smooth cover over the upper part of the wheel well has a pretty large effect on drag.
-Narrower tires - this is commonly used by OEMs on their high economy cars. Check out the BMW i3 - it's 3300lbs and runs 155 width tires.
-Air curtains - Julian Edgar has posted about these recently, and there's a lot of evidence they work. OEMs are using these as well in newer vehicles.
There are probably other items. Post up anything you make!
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