You didn't mention the age of truck or engine size. I seriously think manufacturers rate the towing capacity of vehicles in such a controlled environment, no wind, flat, smooth road, etc.
I towed a 21-foot 2012 FunFinder 210UDS with it. And the Titan was rated for about 6,500-7,400 pounds. It performed well on flat interstates. Headed through the plains into a head wind-struggled. Rockies, Cascades, etc.-wore that truck out. Seriously underpowered.
Just know what you're getting into, and what to expect.
2012 FunFinder X 210UDS
4-68 watt flexible panels; 1-215 glass panel/4 Battle Born LiFePO4 Deep Cycle Batteries
2014 Ford F350 6.7 Liter PowerStroke V-8 Diesel longbed, SRW