The longer the trailer is in relation to the tow vehicle the more the trailer will wag the tow vehicle. So a shorter trailer, something in the 22-24' range is much better than a 28-30 foot trailer.
The 7000 pound max tow weight is also based on the GCWR (combined weight of the truck & trailer). That means when you load the passengers and gear in the truck it may reduce the max weight of the trailer you can pull.
There is also the max amount of rear axle weight, so you must consider the tongue weight of the trailer. The tongue weight should be about 12% (but not less than 10%) of the loaded trailer weight. With a 7000 pound trailer, that would be about 840 pounds, probably more than the axle will handle. Also don't forget to add the weight of the equalizing hitch you put on the back of the truck to hook the trailer to. Passengers and gear also add weight to the rear axle.
A note about tongue weight, if the weight is too little, say 8%-9% of the trailer weight you are in danger of a passing vehicle causing the trailer to start to sway. Also road and wind conditions can cause the trailer to start to sway. If the sway is not, almost immediately, controlled, it will go out of control and result in a serious and sometimes deadly accident.
