I inspected a bunch of these for the prison system. The typical installation was a service disconnect on a post next to the unit, 4 wire feeder to the panel in the unit and a solid ground electrode conductor from the home panel to the ground under the unit that picked up a lug on the frame along the way. The commercial trailers used as office space always had an 8ga factory installed that bonded the frame. YMMV on residential units.
"Trailers" really come in at least 3 varieties.
There is the travel trailer which is cord and plug connected and stays on wheels
You have the "HUD" approved manufactured home that is basically like a house. The service equipment may actually be in this unit.
You also have a "park model" RV that gets the wheels taken off and sits like a manufactured home but exploits a loophole in the law that allows it to avoid HUD standards. These get connected to a post mounted disconnect. They are hard to get permitted in Florida these days.
I imagine there are variations in between in other places but Florida has made it hard to have anything that won't meet the wind code.
In reality, once you put in all the tie downs the Florida law requires these things are very well "grounded" in any case but we still want to see a rod.
