They will buy some, to test, so they can get the jump on their competitors. If you are Walmart with a zillion trucks spending several zillion on diesel every year, buying 6 or even 66 as a test isn't exactly risking the business when the payback might be huge. As in saving 2/3 your diesel costs.
Trucking companies arent dummies, if its cheaper determined by relatively quick ROI (2-3 years), they will buy it like its going out of fashion. Its not like a consumer making a point by buying a car that loses money, trucking companies make hard boiled decisions based on real numbers not fashion so all the naysayers here wittering on about Tesla groupies are missing the point, if these hard bitten numbers people are buying its real.
So, adjust the original posts terminally flawed maths that forgets that diesel engines are 100% efficient, and even from those figures, its a clear money saving proposition, with massive ROI at certain mileages and travel distances.
Whether they can be produced on time, at the advertised cost, in the quantities needed, that remains to be seen. If they can, its a slam dunk.