variable-roof semi-trailer
 

Here's a link to a German patent for an articulated-roof semi-trailer which could morph into a more streamlined contour when dead-heading between loads,or as loads were delivered incrementally,as trailer space allowed.EP2529966A1 LKW-Anhänger mit aerodynamisch variabler und optimierter Form | Patent

That was the basic idea for an RV, it lifts up to make space when parked.
Should work too for cargo trucks.
Just as with the RV idea, the 'challenge' is to get a watertight seal for the flexible area. It can't leak or need a lot of maintainance.

Even if the truck wasn't empty often times they aren't completely full either. I believe they load from the front back so whatever empty space they might have would be where they could collapse the roof after loading.
Lots of slide in truck campers have a pop up roof, as do some towables. They are usually toward the low end of pricing. RVs cost so much new and are such a luxury in the first place for most people, mpg doesn't start to enter their mind until years later and probably the 2nd or 3rd owner and it's too late then.

That's what I have brewing in my brain for the last 5 years or so. Why haul around a draggy box of air?:D

RV
 

The Alto RV travel trailer is an example of a variable-roof concept
https://s-media-cache-ak0.pinimg.com...1649c5d6be.jpg

What troubles me with that design is trying to keep rain out of those long joints. It is the opposite of my design.