Sequoia owner here. Like others have said, you may want to consider LT tires. I bought a set of 18" take-off wheels (Tundra 5 lug wheels match exactly) for putting a set of LT-C tires on. I've only towed with LT tires and am happy with the experience. I'm not sure if LT is available in the 275/55/20 size, you'd have to check tirerack.com or other online tire sites.
A little more info: I've towed my 5.5K travel trailer (12.5K combined at the scale) with my 2011 Sequoia for 8 seasons, 50 or so round trips through the Colorado mountains. Performance is excellent up and down the hills.
