While I haven't had a chance look up your VIN yet your recent pictures tell me a lot.
#1 mod is to unbolt that huge bumper / grill guard and sell it to someone that wants to kill their fuel economy. It not only likely weighs hundreds of pounds but everything behind that is dirty air. This needs to go:
#2. Put the air dam back on or make your own to connect your wheel spats. Don't know if you had the 6-4-6 air dam or 4-4-4 air dam but lower is better. (Those numbers are ground clearance on the left, center, and right side of the air dam
This ebay one is odd. It shows pieces for the 6-4-6 but the 4-4-4- on the truck
#3. Reduce the grill intake size. You can make your own or use a commercial one designed for winter as a starting point. Of course this depends on terrain, temperature, and load but the factory grill opening is worst case cooling requirements: fully loaded, driving up steep inclines in death valley. Keep an eye on the temp gauge.
#4 Your side skirts are good. You should have had some from the factory.
Again lower is better, just don't let them touch the ground when aired down. (In winter they can freeze to the ground and then rip off or break your side fairings.) You can also close up the front wheel openings more like the factory has done. Close to the tire is better.
#5 Mirrors: You already mentioned you took off the door look down mirror. Removing the hood mounted mirrors will reduce aerodynamic drag a couple of percent. As you know but for others - the truck will still have 4 mirrors and be perfectly legal.
#6 Fairing extensions: Yours may help or hurt. The stock side fairings on your truck are more aerodynamic than the previous version that when full wheel to wheel. A lot of work went into getting the kick out angle right
#7: Cab extensions. The sides are optimized but take a look at extending the roof. (Some have this from the factory) You want as close as possible but still allowing turning with the 5th wheel adjusted forward.
#8 Fill the gap between the hood and bumper in the front. You could test with tape but Pipe insulation will be a more permanent solution. (Supertruck 2 has these)
#9 If you want to get real creative the next big aero issue is the negative area between the door and top step. You can look to Mexico for inspiration. Drivers there attach an extension to the bottom of the door. They do it to help prevent hijackings but the aero benefit is there too. (SuperTruck 2 has these too)
Things that cost nothing: Keep your 5th wheel adjusted as far forward as you can and still keep the axle weights legal. The smaller the trailer gap the better. (Supertruck 2 has extensions that fold out at speed to close the gap and then fold back at slow speed to allow turns)
As mentioned above - keep you tires aired up
SuperTruck 2 is here if you are interested:
https://freightliner.com/why-freight...supertruck-ii/ Some good ideas from a $50 million one-off truck.
