One idea for a possible 'best' mode of transportation would be by rail. Two locomotives and around 2-3 cars would be ideal (I could break it down more if you want). It would not be hard to get a rail map of the whole U.S. or a crash course with some books to learn how to operate. There are also railheads into every large military base in the country, which are great re-supply points.
Trains have an extreme strategic advantage. Very easy to defend, can mow through anything, have plenty of storage, and can travel very long distances.
Long term self-sustainment is the most important aspect when thinking of the "endgame", this can easily be achieved with a couple locomotives and railcars.