VOZ, the limiting weight factor for my Tundra is gross COMBINED weight rating - the total of the truck plus trailer plus everything else. If I add the weight of my truck plus the total advertised 'tow rating', I'm WAY over my gross combined weight rating. All your various weight limits are on the driver door sticker or owners manual. Some depend on your engine, transmission, 2WD vs 4WD, etc.
With respect to 'which mods first'. My answer is "none". Get your trailer and go camping. Decide after a year of camping what you want to change.