Check your state laws, there is often a combined length maximum. As I recall, typical for a tractor and semi-trailer is about 75 feet. (I haven't look at that for a while.) That's pretty long, especially in the urban NE or in an RV park - you might have a hell of a time getting around. With a typical 5er, the wheels are mounted so far forward that the swing of the rear end is likely to hit things.
For a single unit, it is typically 45 feet.