has a lot of information about CDL.
Basically, if you intend to use your vehicle for commercial purposes, and it falls into the scope of a commercial vehicle, then you are required to have a CDL.
If your vehicle is for private use only, and not for hire, then you generally don't have to have a CDL. Some states do requires a special license class that is either the CDL or a non commercial equivalent. You will have to check your state's specific laws concerning commercial vehicles and what is exempt from being classified as commercial.
For virginia, as an example, you have to look in the code section dealing with commercial vehicles, and the definition of commercial vehicles, to learn that there are exemptions. Specifically: operation of a vehicle for personal use only, such as a recreational vehicle or truck to move your personal belongings, operation of a fire truck or other emergency vehicle, operation of a farm vehicle that meets the requirements of a farm vehicle, Operation of a vehicle for military purposes while the operator is on active duty carrier.
contains a very basic definition of a commercial vehicle, which will pretty much be the same across the country.
I have a thomas rear engine pusher with 3208, GVWR of 33,280. It has air brakes, and currently can sit 4 people including the driver. It is titled and registered as a motorhome. I drive it with the same license class I've had since I turned 16. I'm also 100% legal that way. I carry liability insurance via GMAC, and pay the tax and tag fees each year. I can get in it at any time and go joy riding if I want (assuming my wallet can stand the fuel hit).
When I bought the bus, it was titled as a "housecar" in NC, which is the same as a motorhome. I paid VA for a temporary trip permit, and printed that out and put it in the back window. I drove from NC to GA, spent the weekend, then drove from GA to VA, and the cops never gave it a second look that I noticed.
Here in VA, you could drive that bus (privately registered), towing that trailer (privately registered), and not need a CDL. In fact, the trailer can exceed 10,000 pounds quite easily, and still not need a CDL. Assuming of course, that there was no commerce involved. As soon as commerce is involved, it becomes commercial, which brings in the federal requirements for a CDL.
I hope this helps,