One potential solution might be to power the trailer, too. I’m thinking along the lines of the way that old trains were dragged by the engine, but modern trains deliver power to the bogeys of each individual carriage.
Electric motors can be small and easily installed, used for acceleration and braking, and the bed of the trailer could sandwich a fair amount of batteries. Electrical hookup for the lights can also be used to give the driver control of the trailer’s motors (heck, the connection for the brakelights themselves can tell the trailer when to brake).
I’m not suggesting someone jerry-rig this, but it seems like an obvious companion product for the new wave of EV trucks. In fact I wouldn’t be surprised if a battery-assist trailer would be beneficial for ICE truck drivers, as well.
[edit] I posted this before I saw others had already suggested the same thing.
