I flat towed my W123 Benz behind my 4x4 Dakota from Seattle back to Ohio when I moved 7 years ago. I wasn't as bad as I thought, but braking from 70 mph took longer than I wanted it to. Extra braking power would have been nice , but I survived without it. Below 75 mph it was stable, but over that and it would "just" start to sway a bit, so I kept it at about 65-70 most of the time. I left the steering wheel unlocked as well. I figured the old Benz had plenty of caster to keep it tracking straight.
A friend gave me a tow bar and I modified the way it attaches. I basically ended up copying this design - NMNA (http://thereceiverextendersadapters.blogspot.com/2011/06/american-tow-bar-universal-tow-bar.html)
I used two 10,000 lb ratchet straps and J hooks to cinch it up to the bumper, with safety chains attached to my truck, checking the straps every time I got fuel or stopped for some reason. I had to plan all my stops so that I could pull straight through, no backing with that set up, but it was easy to disassemble if I needed to.
It is do able, but as others have said, there are some trade-offs no matter what route you take. Largest concern would be braking, if I were to do that again.
