Welcome ! Your smart for asking ahead of buying. Do not listen to your salesman, he’s trying to make sales. You have to do your own research and here’s a start.
https://youtu.be/qwFLOBrADBs
This is the video that I watch 2 yrs ago and it paid off. I have a 2019 GMC 3500 diesel pulling a 2020 3120rl (actual trailer weight is 15860 lbs and pin weight is 3780lbs) and I’m still 140 lbs over weight capacity. Watch some YouTube videos on Rv trailer weights and you will eventually figure it out. Good luck and safe travels !