Have posted before on this but thought I would give an update as this was my longest and heaviest tow to date. I have a 3500 pound dry weight toy hauler loaded with about 1000 pounds of stuff, including a Harley dyna super glide. Trip was from central NJ to the Shenandoah Valley in Virginia. I have a 2012 4dr sport with manual trans and max tow package (3.73 rear). I also have dual axle brakes on the trailer, a brake controller and friction sway control. Except for the significant need for power, the trailer towed like a dream. No wandering, no sway. I kept it around 60-65 and felt like I had very good control. It is only possible to go about 73mph. Just not enough power to go any faster. My mpg was 11.5. What I have found in previous trips is that 60mph gives 12mpg and 70mph gives about 11mpg. It towed great, but I don't think I would want to do it with the old engine or with lower numbered gears. Also, I have not noticed much difference compared to towing the trailer empty vs towing it loaded. I guess the box towing a bigger box just has some aero limitations, regardless of weight.
I also should mention the weight distribution is an absolute must have for towing this weight. Also, there seems to be a sweet spot for the amount of tension. Each of my last few trips I have tried different links in the chain. Getting it right improves drivability, handling, and control. I was passed by many big rigs. There was a very slight vacuum pull but nothing had at all. Box trucks seem to have a worse pull when they are passing you compared to tractor trailers.
Overloading the vehicle by 1000 lbs and driving 70 miles per hour; congratulations sir, you are a braver man than I.
I've done my research and am comfortable towing that much with the precautions I have taken. The same gas powered 2012 jeep in countries outside the US is rated for at least 4400 pounds. The rating is not so scientific. I would definitely not tow this trailer with a jeep liberty even though they are rated for 5000 pounds towing. Also, when I hit 70 it was on a newly paved highway with essentially no traffic and the speed limit was 70. Yes, I am aware of liability issues going over the US rated tow limit. Let's not make this thread about those issues. I wrote this thread in order to help someone make an informed decision about their towing after reading about my positive experiences.