2014 Summit 5.7 4wd 20" tires swapped to 18", added all skid plates
I have towed essentially the identical setup. Towed up to 75 mph on what passes for steep grades on modern highways, at below sea level and at 10,000 feet. At temps as high as 115 going up the steep grades. My 2014 5.7 had no problem at handling all of it, temp never went up more than 10 degrees, no sway ever, no problem accelerating, cruise control plus the 8 speed maintained speed up hill and down. Did one trip with a loaded tandem axle car trailer. no WDH and only surge brakes, trailer weight was probably about 4500. No problems at any speed. I think one of the keys is the tandem axle.I've been pulling a 19' travel trailer for a couple years now with a 2016 Limited diesel, factory tow package, weight distributing hitch with sway control, and it tows great.
I haven't actually done this, but for payload capacity, one of these days I'm going to fill the GC up with gas and leave in my regular gear that is tucked away around the spare and such and go get it weighed. Subtract that from my GVWR, and call that my real world payload capacity. I've not heard of anyone ever getting in trouble for being over some payload number if they are under GVWR and under axle load ratings as well.
My anecdotal towing experience.
The trailer is a tandem axle with a gross trailer weight limit of 6200lbs. I've had the trailer up around 5000lbs for longer trips with the gear for the whole family and a full water tank when headed out in the boonies. That puts me right at my GVWR with people and tongue weight, and I don't load up any gear in the GC. Fortunately I have this trailer behind me to stow all the gear. The tandem axle really helps with the stability, and the 4 wheel brakes are nice. I haven't had any sway problems on the highway at 55-65mph and passing or being passed by other large vehicles. I get a little side push, but nothing that is worrisome. The trailer is also 7' wide which I think really helps by not punching as big of a hole in the air as an 8' would, and I really like it when on the narrow mountain roads in the Pacific Northwest where we live and do most of our traveling.
Hope that helps your research.