I think we are misunderstanding the OP. A "Smart Trailer" is a "swivel wheel trailer" AKA an "extension". It is not a trailer in the usual sense. It has two attachment points, and either a single swivel wheel or two swivel wheels. They are easy to back up, and legal in all states, as far as I know, as long as you don't exceed the maximum length restrictions.
More info here:
https://smarttrailers.com/