I don't usually do this, but I'm puzzled. Since you seem to think I might not know of what I speak let me disect your original post.
You will hear many horror stories if you keep trucking. Most of them have a tiny bit of truth in them, the rest is embelishment.
I have been trucking 43 years now, and I have heard most of them time and again. Always happening to someone known to the spinner of the yarn. For example, the 55mph speeding ticket in Ohio or the 120mph truck that got shipped out of Arizona on a train.
When was this? $4.00 a gallon gas is not a common price.
For a fake $20? More like you may have exposed a trunk full of it. My wife, (the cop that just retired after 28 years), had many money laundering calls, gas drive off's, bad checks, etc. She always handled them all alone, unless the suspect became combative.
May I ask how long it took to have the "feds" respond to your location? They are not usually readily available, unless they were already investigating something in the immediate area.
"Box truck"? Where in the world were you? Usually only large metro area's respond with a "paddy wagon", and that is most often for transport, not for interrogation.
I truly believe there was most likely no US Marshal present. Unless he was there laughing as he bought coffee and donuts. Sounds more like the Drug Task Force in NC, they target individuals, and vehicles that fit a profile.
Here is the primary misson of the US Marshal's and it is why I question one being there.
The Marshals Service occupies a uniquely central position in the federal justice system. It is the enforcement arm of the federal courts, involved in virtually every federal law enforcement initiative.
•Presidentially appointed, U.S. marshals direct the activities of 94 districts — one for each federal judicial district.
•More than 3,950 deputy U.S. marshals and criminal investigators form the backbone of the agency. Among their many duties, they protect the federal judiciary, apprehend federal fugitives, seize property acquired by criminals through illegal activities, house and transport federal prisoners and operate the Witness Security Program.
Overview of the U.S. Marshals Service
I truly think, you were bamboozled, it is doubtful there was ever a federal official in attendance. But what I think is not important, the facts as you presented seem a bit out of sync with things as I have experienced them.
I wish you well, and I do believe you will fit right in with the counter crowd at the truckstops.