If the "trailer" is served by a hard wire (chapter 3) feeder 550.33 referencing 250.32(B) requires that ground rod.
Then you get into parsing the difference between a mobile home and a manufactured home. Most of the AHJs around here do not make that distinction. If you take the wheels off and tie it down, it is a manufactured home and treated, pretty much like, any other structure.
We are not really setting enough of them to create a precedent these days.
