In my area, the state patrol have a commercial division unit specially trained for their job. They are still LEO's but with specialized training in commercial vehicle compliance and safety.
DOT can be at the state and federal level which makes it mess if you're not sure which you are dealing with. Although the Fed's may set standards, the states have a lot of latitude on what they will accept on their roads. I have always seen LEO's enforcing laws based on DOT and have not seen DOT directly enforcing something they have defined.