2006 Toyota Tundra towing capacity question

I towed around 6500 lbs with our old 2000 Tundra 4.7 V8 for around 4 years. I always felt like the truck was towing at its limit. It was fine on the flats but slow on the hills. We mostly did local camping but once took a 1500 mile trip. While not ideal, the truck could handle it. Later upgraded to second generation Tundra. Yes, you can do it, but a bigger truck would handle it better.