There is something like a 70-90% parts commonality between a sleeper cabover and a conventional tractor, indeed - the only parts that differ are body, frame, and a couple interior pieces - even drivetrains are shared - and commonly shared between europe and NA - though European engines do not have the best reputation among US drivers (neither the Volvo or Mercedes Class 8 engine is well thought of - parts are expensive and take a long time, and sadly they're needed with alarming frequency). Indeed, many interior parts can be shared between a cabover and conventional truck, including the dash, and most of the interior appointments.
I typically do this in incognito windows as I do not always want for instance English and Italian results.
For example, a 2mx2mx0.5m pack can be slotted between the cabin and trailer.
A 2mx6mx0.25cm can be laid on the roof ?
Etc..
(I have seen demos of an Australian truck where the fossil engine is removed, and the pack is inserted from the front.)
There is a maximum vehicle length though, and a maximum weight, the former ensures vehicle maneuverability, the latter serves road stability, bridge specifications, optimized no-waste parking provisions for pass-through parking spots in said truck stops edit: and ferry capacity. Ireland, Finland are connected by Ferry.
Manufacturing is pretty much internationalized, reusing parts and designs globally. Freightliner and Daimler are one company, for example.
Fuel economy is indeed a point, albeit the combo of trailer and truck define the overall fuel economy. As the trailer usually is equivalent to a standard 40ft container, the ideal truck has a boring standard form, too.
The text is German, the pictures are self explanatory-ish though wrt optimal and sub optimal wind résistance: https://en.vda.de/dam/vda/publications/FAT-SCHRIFTENREIHE%20...