I bet it's not hard to tell a Peterbilt with trailer from a Honda Civic. They OCR the license plate number in real time, which is a whole lot more sophisticated of a problem than what I suggested.
OCR of license plate characters is far easier than distinguishing specific vehicle and weight classes. Fortunately, the license plate is connected to a database that tells you much of what you want anyway. Couple that with weight sensors (to count axles), and you've got the data you need for tolling without doing any "hard" visual recognition.