> I always assumed the future of electric trucking would involve little battery trailers that could be quickly swapped out on a rolling basis by fleets of trucks.

Maybe the future, but right now, I suspect the battery energy density is not forgiving enough for the battery to just be a carry load. Increasingly, the battery module is part of the structure of the vehicle and further works as weight to increase traction close to the drivetrain. There is also a lot of nitty-gritty details to do with safety, efficiency and cooling that makes moving the battery away from the Semi itself not a viable option ATM.

Also keep in mind that at least for consumer cars, Tesla did try battery swap stations but nobody seemed to show any interest so they canned it.