Some real world #s for you as I just got back to my home in SC after towing our camper down from our cabin in central NY with a few stops in between. I stopped in at a cat scale near home here with this thread in mind, and wondering just how overloaded I might be. Having packed up the cabin for winter, we somehow managed to be travelling with a bit of stuff in the truck bed.
My 2017 ram 1500 truck specs. gvwr is 6900 lbs, gawr front and rear axles are 3900 lbs each, combined weight of occupants and cargo maximum is 1378 lbs. gcwr is 13800.
First lesson I learned when I weighed my truck alone about a year or so ago is that its weight is 5880 lbs. with me in it and not much else. 6900-5880=1020. That's the amount I can add to it without going over gvwr and is a far cry from 1378. I weigh about 162, not 300, if you're wondering.
My trailer is a Grand Design Imagine 22mle that is about 26' overall external dimensions. gvwr is 6995 and dry weight (uvw) is 5184. Advertised tongue weight is 608 lbs. When I've actually checked this on a scale, it's been between 800 and 960 depending on how I have it loaded. FWIW, 15% of gvwr is 1049, so worst case scenario is a lot closer to reality than advertised tongue weight.
I do carry 2x 47 lb Honda generators in the front pass thru storage area, and have 2x 6v golf cart batteries on the tongue and a pair of 20 lb propane tanks there too, and a real mattress inside, so I do carry a bit of extra tongue weight. I could move the generators to the back of the trailer, but I'm lazy and don't like lugging them back and forth each stop. This is reality when you get a trailer tweaked to how you want it.
I'm attaching the scale ticket from today. As you can see, I'm about 260 lbs over gvwr of the truck. gawr of both axles is well below the max, so I'm not really concerned that I'm over gvwr, and the total is less than gcwr. The weight distribution hitch is set up darn near perfect for my setup, and the truck tows this trailer just fine. You rarely notice it's even back there most of the time. No issues up and down the mountains. I just set the cruise to 65 and let the truck do its thing. No issues with sway or being pushed around by the wind from the semi's passing. The truck is not squatted at all in the rear. Also, the trailer weight is no where near its gvwr. Some number crunchers on the forums would say I'm way overloaded and need a 3/4 ton. I see no reason to move up to one. I'm usually right at my gvwr when I'm not carrying a bunch of extra stuff in the truck bed.
So my take home message from this long winded post is you probably don't have as much payload capacity as you think you do (weigh your truck to find out), don't believe or make plans based on manufacturers listed dry weight or tongue weight. 15% of gvwr is really a much better # to plan with. The truck being overloaded by gvwr is only part of the picture. How the weight is balanced is more important. You could be under gvwr and have a rear axle overloaded due to an improperly set up weight distribution hitch, and the trailer could tow like crap because of it.