I have a 2016 GH 31ds with the 6 speed, and my toad weighs in at about 4000lbs. I haven't been on any big hills yet , but it pulls my toad just fine. Actually not much difference than when I am not dragging the toad. I put the trans in tow/haul mode and it works very well. Will even downshift more when slowing down to ease the work on the brakes. Trans shifts very smoothly and plenty of power.
Tom & Deb
2016 Greyhawk 31DS
2016 Chevy Equinox LX
Blue Ox Baseplate w/Aventa LX Tow Bar & KarGard II Deflector
Blue Ox Patriot Brake