Also depending on the type of operation, the trailer owner may not care about how much diesel the truck owner burns.
One benefit though is that the trailers could slow charge off of a basic 240V charger while parked at a loading dock between uses.
On the flip side, trailers spend more time parked at warehouses where they can charge leisurely.
Eliminating the need for fast charging could extend battery life or even allow for the use of cheaper battery technologies. Maybe we'll even see a future where the tractor draws most of its power from the trailer.
Then there's the other things you could do with a trailer that has always-on power. Climate control and realtime GPS tracking (and other telemetry) come to mind.
There by getting the trailer owner to care if they have loads hauled regularly.
Also, I imagine it's easier to find extra space for batteries in the the trailer than the tractor.
Regenerative breaking seems like a more important improvement though.
The trailers could also charge off the alternator of the truck itself which could help to calm concerns of long charging times at stationary sources. The percent gains would probably be in the single digits and would demand a bit more from the engine but maybe there's an optimal balance there.
Charging a hybrid truck's lithium batter at 1 KW is nowhere near enough power to be useful.
If you want to have the diesel engine charge the battery that means a total redesign if the truck, its engine, and its power train.
If you optimize for a more efficient Regen using a proper purpose driven 'alternator', and only do it when the truck is slowing down, that would be much better. Aka, current hybrid tech.
Now, if for some reason the engine was idling for no reason other than to keep a dome light on for an hour, then yes you could generate 100 watts as fast as possible and then turn the entire engine off, leaving the dome light lit for several hours off that 100watts you made. That is what a current hybrid does, again.
But if the engine has to idle for other reasons, even laziness, making power while it idles is simply using extra fuel to make that power.
The trailer can charge next time you need to slow down or go down a hill and engage the regen braking.
One interesting thought though: if these trailers could implement thrust vectoring and other safety measures(like adjacent vehicle detection), it may be realistic to run longer trains than can be done with dumb trailers that tend to be unhandleable beyond 2-long. This may effect a reduction in the trailer pool ratio, increasing efficiency even more.
I grew up in the Columbia River gorge and I remember driving alongside triple-trailer pulls and it was terrifying to attempt passing them when the wind was gusty. I'm confident everyone that frequented that corridor could regale you with tales of being almost run off the road by one of these behemoths at least once.