I would consider the trailer as portable equipment since it has a cord and attachment plug. Even though the trailer was probably manufactured to the stringent standards of MHMA and other required standards (which by the way are now a lot stricter than they were twenty and thirty years ago when I bought my first mobile home).
The cord and plug have two phase conductors, a neutral and a ground. The receptacle supplying power to the unit is deemed as grounded and providing a ground return path. Having said all of that, I think I would insure the shell and frame are electrically connected to the ground in the cable. An ohmmeter check should suffice. If your meter is in the truck a couple of hundred feet away then a scrap piece of #10 connected to the frame and to the receptacle conduit should provide a suitable ground path for fault currents. A separate ground rod may set up a situation with circulating ground currents, and may cause additional problems.
