Far and away the largest road use tax is included in the price per gallon.
Currently, only about 2% of trucks are inspected during any part of the loads movement, and the addition of the Mexican trucks will just exacerbate this problem. In addition, I could tell you about the proclivity of the FL DOT to let the non-English speaking drivers ( which is lillegal in the first place) slide on violations due to the language barrier and the hassle that causes for the DOT officer (they are required to find an interpreter), but that is another, although related story.
This whole issue is right at the heart of the US economy, and is going to cause problems like you won't believe until the fallout starts. Truckers are an independent bunch, and will do whatever it takes to keep their business and support their families. This thing is going to get physical before it's over.