For towing, the primary consideration would be load rating. I would want at least what max RAWR covers in an LT tire (ie: if the rear axle is rated at 6000lb on the door sticker, I want at least a 3000lb rated tire).
Best MPG is typically going to be a highway design but if you do some offroad work, you probably want to trade off for a slightly more aggressive tread (but nothing crazy). But unless you get into extreme off road tires, I wouldn't expect much impact on MPG regardless of choice. You certainly shouldn't be seeing 2-3MPG difference based on tires unless something is wrong.
I know people swear by brands but never had an issue with off brand tires of the proper rating. Always got good life and no issues. Only failure in the last 15yrs of towing was when a leaf spring on the trailer failed and the broken end swung over and cut into the sidewall of the trailer tire.