Those unibody cars we had easily lasted 12-years or 200k. And those trailers were each in excess of 7,000-lbs. 1,100-lb tongue weight. Which is no challenge if one understands how a weight-distribution hitch works. (97% don't. You won't find it on Stupid Tube). Low center of gravity plus better suspension & steering sophistication are what's wanted.
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I can't argue with what you said but I have to wonder what unibody car you used that could tow in excess of 7k.
Not here in the states, but in Australia I know guys doing it with Caprices. Long wheel base 355hp 400 ft/lbs of torque, big brakes.
I have no doubt my car will move 7500 lbs, just not 32 feet. Under 25? Most likely.