2006 Toyota Tundra towing capacity question

Update: We went on a spontaneous trip, driving over 3 hours through lots of hills and on many windy roads. With all four of us, plus the dogs (600+ lbs total), the truck strained. It never felt unsafe except for a very steep decline during which I crawled along and gritted my teeth. Our load was light, including totally empty tanks--no LP even.

I know future trips will be closer to home and on more level roads, so I'm not concerned, but I feel like I just found the truck's limit. It was a good learning experience.