I would think if a series hybrid were more efficient than a conventional mechanical drive and transmission, it would already have been adopted. Trucking companies will spend money even for small gains in fuel efficiency because they drive so many miles.

I also think that regen braking in semis would be trickier because a lot of the braking happens on the trailer wheels so you would need to fit all the trailers with regenerative braking equipment.

Long haul trucks fairly hit the breaks as that ends up costing a lot of money. However, a truck specifically for mountainous areas may benifit from a hybrid transmission.