I read your post several times and have no idea what you're talking about. No Tundra that I'm aware of comes with a 20 gal. tank. My old Tundra Crew Max that I sold at 106K miles (26 gal tank btw) was flawless and towed 8,500 lbs, which I admit is 3/4 ton territory which is why I upgraded to the 2500 Ram CTD. Last thing I would consider as an upgrade over a Tundra is a Titan. Power is less and mpg sucks big time. You already have a bulletproof truck in the Tundra so why would you want to trade down to something less capable?
I did put LT tires on my Tundra. For the small amount of towing you do, I would stick with the Tundra. If you want to "upgrade", go to a 3/4 ton. Many like the newer Ram 2500's because they have the coil spring rear suspension and ride as good as your Tundra.