Everything I can find is that the 300 mile version uses a 500 Kwh pack. The 100 Kwh pack in my X weighs about 1,100 lbs. If you go by that, the semi pack should weigh about 5,500 lbs. Lack of a cast iron engine, transmission, 12V batteries and fuel, will go a ways to canceling out a part of that 5,500 lbs.
Not to worry though. Bill Gates has been quoted a saying such things are impossible and the head of Mercedes truck division said the same laws of physics apply in CA as they do in Germany so it's impossible.