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

My friend has a 6 month old Silverado 4x4 crew cab with the 5.3 and towing package. We towed two large motorcycles on a Featherlite aluminum trailer from Virginia to Florida. The total weight was around
26800 lbs and I was very surprised how poorly his truck towed that small load. Didnt have very good pickup and it seemed like it was always straining to keep up speed on the interstate. Kept downshifting and running the RPMs up real high. I expected a lot better performance then what it had.