There's essentially no efficiency penalty for going up a grade at a faster speed (higher power output) with an EV except for the aero penalty of the higher speed. That equally applies to flat travel though, so there's no reason to slow down for a hill with an EV if it's capable of maintaining speed.
Gasoline vehicles tend to be most efficient at about 80% torque, and 50% of rated horsepower. I'm not sure about the BSFC for diesels, but they can probably be very efficient to the upper portion of their power rating.
The EV trucks would indeed recapture much of the energy spent climbing the grade.
EV drivers report little difference in overall efficiency with regard to climbing grades as long as the destination elevation is approximately the same as the departing location. Efficiency plummets while climbing, and is recovered on the way back down.
As an aside, many truckers are paid per mile. That means when they are going slower than the speed limit, they are getting paid less. Hills can be seen as a reduction in pay... unless you can maintain speed.
Semis in Oregon average about 60 MPH with a speed limit of 55.
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