You can tow right up to whatever limits are dictated by the manufacturer, as stated above payload is usually exceeded well before others are reached. I've seen many construction crews (some that have done work for us) exceed their tow vehicles ratings, but they are not hauling 12' high, 30'+ square payloads at interstate speeds.
Just a plain fact that the closer you are to your max capacities the lesser your safety margins are, no doubt towing comfort is much more pleasurable when you are below stated capacities. Our rig can vary as much as 2500# depending on what toys or how much water we are carrying, the difference in towing comfort is night and day.