Greg, I know the situation from personal experience in Nevada, and I am told that Indiana has the same rules.
To be clear, there are three professionals being discussed here: the licensed electrical contractor, the licensed trailer setter, and the electrician licensed to work on trailers.
The EC license does NOT permit you to so much as change a light bulb in a trailer.
The trailer setter is licensed, not by the Contractors' board, but by the "Mobile Home division." He's allowed to do the hook-up.
The "Mobile Home Division" also licenses electricians to work on trailers. Such electricians can hook up the trailer. This licensing is in addition to, and nearly as expensive as, the license issued by the Contractors' board.
Oddly enough, this change was made to Nevada law after I had my EC licence for about six months.
