BandH -
I have a 2010 F-150, 5.4L, with tow package, when I researched them I found that the engine size was only one factor in the tow rating. Some others were, rear axle ratio, springs, tires, body length, brakes, vehicle weight, hitch setup and the inclusion of a tow package.
Best I remember, depending on the options I choose for my F-150, the tow ratings were from as little as 5500 to as much as 10,200 pounds. I don't remember the exact numbers. With the options and model I choose, I ended up with 9,800 pound rating. Most of what I've found says that the realistic towing amounts are 75 percent of the rated amount. So for me, towing in a mixed environment, I don't want more then 7000 pounds, and part of that is going to be the cargo in the truck itself.
I know some people push the rating a lot closer, but I tend to be conservative. If you use the link to Camping Life Magazine, found at the top of this thread, it has a great tool that’ll get you pretty close to the rated amount for your vehicle.
Hopefully we'll get some input from some other members on this too. Here's something that you don't want to have happen, of course I think this guy has more then one problem.
YouTube - Small Car towing caravan crashes
Neal