I am towing our 2014 Serenity 25FB with a 2008 Range Rover HSE with the Westfalia hitch rated for 770 lbs TW and 7716 lbs trailer weight. AS sold and installed a Blue Ox Sway Pro with 1,000 lbs bars. Land Rover installed the brake controller and the service techs said that the RR should have no problem with the weights and WD for the Serenity. We have 600 miles so far with the highest elevation of 1,500' and so far no problem but you will know the AS is there on long grades if you can't start out at 55 mph.
2014 25FB International Serenity, Blue OX Sway Pro,
2016 F350 4x4 Ultimate Lariat crew cab SRW, LWB, 6.7 PSD, 20" wheels, Ingot Silver Metallic, DiamondBack tonneau cover, TrailFX wheel-to-wheel step bars.