What you can get away with flat towing also depends a lot on the terrain.
Here in VA we have mountains to contend with. Out west a lot of places are seriously flat and you might think it's easier to flat tow something- until you get in a pickle and then it doesn't matter where you are.
Dragging along a lot of weight without additional brakes and with a tendency to steer on it's own can get interesting.
For my trip and Tim's GMC mentioned above the towing truck weighed 10,000 lb empty and I had another couple thousand pounds of things on the bed for most of the trip. Not sure what Tim's truck weighed, perhaps similar? it is quite long.
Here's the tow bar I used, military surplus for things about the size of a 2.5 ton truck.
stretching it out quite long I think helped the pulling truck to control the towed one better than a short hitch would have.
https://goo.gl/photos/AAnLoqp9iEAsnqzFASometimes going over a bump on a bridge for example the GMC would still wiggle and jerk the ford around. Ordinarily in that case you'd gently apply the trailer brakes to straighten things out again from the back, but with no brakes on the towed vehicle it can get interesting.. I was able to mash the gas and yank it all straight from the front quite effectively. The ford had a Detroit Diesel 6V53 with twin turbos, it was something else to drive.. and listen to. Wish I could have been on the side of the road listening from a distance as it pulled the grades on I-90 in Idaho, it was singing/screaming pulled even with and slowly passing a semi truck on a long grade doing all it could at about 45-50 MPH. That was a fun trip.
