The 6 speed behind the 6.0L is a GM 6L90E, not an Allison. That being said, definitely go with the 4.10's as the trailer weight requires it. Not sure if you are looking at new or used trucks but if you are looking at new, I would recommend going with a used diesel and not even worry about mileage. It will be 30-50% better than the gasser all day, everyday.
2017 28BHBE Kitchen skylight, remote control and Aluminum wheels hitched by ProPride 3P
2012 Jayco Jay Flight 26BH traded in
2013 Toyota Tundra DC 4x4, 5.7, 6 speed 4.30 gears