I have a 2018 Limited 4x4 with 6'+ bed and I pull a 7500# travel trailer with no problem at all. I can run 70 mph for hours; plus I've never had a problem with oil overheating. I would not hesitate to go up to 8000 based on my experience, but I'm also running both an engine and trans tune and that reportedly helps. Have never towed without the tunes.
There is basically no difference from 2015 to 2018. What is your axle ratio? do you have air suspension? load equalizing hitch?will you be pulling a lot of passes? there are a lot of variables...
I know my transmission has been run at 198 degrees all the time within 15 miles when warmed up and I dont know if that's normal and that's only at 75mph
I have a 16 and my transmission will average 190 and I live in VT. I don't use the highway often and don't exceed 70mph, most of my driving is around 50-55mph on secondary roads. My transmission temp will escalate when towing my 6000 pound travel trailer or if I'm pulling big hills. The temperature never exceeded 210.