SRW v. DRW

So I do now own a Ram 3500 Longhorn Mega cab diesel. I'm currently using it to tow my travel trailer. Want to move to a Heartland Gateway 3650bh which grosses at 15500, dry at 13025 and pin weight at 2020. I know the truck can both handle towing weight and payload (hitch) weight.

As others say it depends on which year model and do you have a SRW or a DRW 3500 truck.
Look on your trucks door post and tell us what your trucks GVWR and RAWR is.
If you have a 3012 and older SRW has a 6200-6500 RAWR you will have to mod the trucks rear suspension to carry more weight.
If your truck is a 2013 and newer truck it has a 7000 RAWR and may have enough suspension for a 3000-3500 lb pin weight.

if I go with 20% of Gross, I'm still under for payload capacity of 5500 on the SRW.

You say in another post you have a SRW and it has a 5500 lb payload. There isn't a SRW out here that has a 5500 lb payload. If your truck has a 5500 lb payload its is a DRW.