Depends on your state. They are all different. If it an RV is where the loop holes are.
Just remember if you are required to have a CDL to drive that combo you can get your CDL in it.
Make sure you are not over your GVWR with the trailer.
Most big trailers have three 6,000lb axles and a dually is not rated for that much weight. Yes, people do it al the time, but that doesn't mean its legal.