Well, to help the mirror issue, you could use a camera setup. By eliminating your mirrors and installing cameras you would significantly reduce drag (I have a bus, and the mirrors are almost as much frontal area as most auto front windows). The advice of cleaning up the front end is good advice as well.
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With a diesel, turbo pressure increases efficiency. Dont lug the engine, but try being more patient on the road instead(i.e. sometimes we pass for no good reason).
As for the trailer, I dont know how much time you spend hauling a load, or not hauling, but it may be worth it to close up the wake of the truck. I am thinking of a wedge like device that goes from height A by the truck and decreases to deck height, or height B over some distance, C. Making it easily collapsible and storable would be the only thing that would be difficult. Good luck!