Coolers, spare tires, air tank, grill, propane tank, clothes, bicycles, tools, water, gas cans, food, friends etc. starts to add weight as well.
Wind, hills, curves, heat, cold, bumps, pot-holes, slow traffic and other things can really affect the engine, brakes, tranny and suspension.
If you're going to be trailering Harleys then I think you might be going for distance.
I've trailered loads with both a 1/2 ton (Suburban current) and a 3/4 ton and the 3/4 ton sure is always nicer for those heavier loads. The wind is where I really appreciated the 3/4 ton suspension and downhill is where I really appreciated the 3/4 ton's heavy duty brakes.
Congrats on finding the trailer you want. Looks like a great find. Enjoy the amenities!