I assume you've got a commercial-type 5th. wheel, not the downsized type used on some RV's? If so, and the gross weight capacity of the tractor/trailer combination exceeds 25,000 pounds, you're going to need to have a Commercial Driver's License, and the truck and trailer will have to run commercial license plates unless it's registered as a farm vehicle in most states. I don'think hauling cars qualifies as transporting "farm" products. You'll also be required to meet federal commercial vehicle inspection standards. Before you get set up with a rig like that, let me suggest that you investigate the licensing and insurance cost of putting it on the road. Your wallet might yell "OUCH"! BTW, there's also a much stricter blood alcohol level threshold for people with a CDL at a DUI checkpoint, even when driving an automobile.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!