A Yahoo Answers post [1] gives an idea of the kind of metrics TSLA will have to soundly beat. Fuel is the biggest cost. Bring that down and you'll get the trucking companies' attention for the pitch.
Not all semis are long-haul; there are local delivery semis that fit into the 2-300 mile range, and that could be an initial market.
[1] https://answers.yahoo.com/question/index?qid=20100327005347A...
If I were swapping batteries, I would have some concern about the condition that battery was in. In the consumer market, it's hard to manage that in a trustful way.
Some shipping networks will allow swappable packs to remain in the possession of one entity. Hopefully, that entity trusts itself.
It doesn't come close. Most semis load 250~300gal diesel and have a fuel economy of 5 to 8mpg[0] so you're probably looking at at least 1000 miles, and possibly > double that depending on the exact model, trip and driving style.
[0] though recent full-aero tractors can reach the low 10s, Volvo advertises 12MPG on their most efficient models
Also more of the tractor can hang underneath the trailer so there's the chance for a larger battery capacity. Like this extended tractor (the semi painted green not the flatbeds behind it) in this image that is 22 feet long. http://www.bigtruckguide.com/wp-content/uploads/2015/01/Sout...