I saw a really small hard-top popup camper on Craigslist the other day, called the Coleman Destiny Roanoke. It looked like it was about 12-ft long, tongue and all. Open it claimed 17 feet. My Festiva has a towing capacity of 1,000-lbs, and the weight of that popup meets that maximum limit EMPTY. Does weight play a factor in classification of a TTT?
I have also seen popup trailers used by motorcycles. They must be TTT's, right? I found them in my search for a <1,000-lb trailer (my wife insists on a standy, and that's going to be difficult for under 1k#), but for the cost, a TD is a much more viable option...