Well, I have told the story before, but will again...I pulled a 15,000 lb dozer on my old trailer (purchased a new one since with heavier GVW) and the trailer weighed 3050 lbs. There is nothing flat here and the brakes on truck were new and trailer functioning well. I only pulled it on back roads and for a few miles, but let me tell you, the stopping distance alone scared heck out of me and I was NEVER over 35 MPH! Not that I couldn't do more, but that I wouldn't. A buddy bought the dozer from my neighbor and I hauled it about 20 miles for him. I was impressed at how well my setup hauled it, but would never do it and feel safe about it daily. I have a 15,000 lb hitch, weight distributing setup for 14,000 lbs and the dozer was able to be moved around on the trailer to get the weight over the axles the way it needed to be, not so easy with wood. At any rate, they'll do it, but I don't think anyone thinking right would recommend it!
I wouldn't want that much weight behind me on a 5th wheel/gooseneck either, but I bought the heaviest "Quality S" hitch I could buy (15,000 lb WD) hitch and never looked back, well, I did look back, but I didn't really need too, the weight shoving me forward was assurance enough I still had a load! The same dozer was hauled behing a SRW F250 for 6 miles, 460 engine and the guy was scared to death to bring it back to it's owner, so I did. When they loaded the dozer on his trailer, it lifted the back wheels of his truck off the ground...I think that made him a little smarter about moving it around. They have a tree trimming business and have since purchased a tri-axle trailer and a dump to pull it around with, so it is a safe haul these days.