how much weight do you tow with 1/2 ton gm 5.3l

My neighbor has basically the same truck but four years newer and has a 6000 pound 29 foot trailer (the box part not including hitch) and he is thinking about moving up a notch in truck. He said it tows fine on calm days but on windy ones the truck screams going into the wind and is all over the road - even with a good WD hitch and sway bars. The weight seems to be fine but the physical size of the trailer (wind area) is the problem. I would go smaller than 29 foot long.