I have a 2014 Ram 2500 with 6.4 Hemi that I bought new.I mostly tow with our diesel pusher motor home but to our local track I use the pickup. My trailer loaded weights around 9800 lbs.
I always use the "tow haul" button and the motor/trans does not go into complete lock up plus the motor stays at a higher rpm at highway speed aprox 65 mph rpm at around 2400rpm
My truck has 3.73 gears . It will downshift if when going up hills but has plenty of power. Fuel mileage is around 8 to 9 mpg. My son just bought a 2018 Ram 2500 with 6.4 Hemi but with 4.10 gears.
His trailer weight around 11000 lbs but he just got it so we have no info yet. I know the towing capacity is around 1800 lbs more with the lower gears but I believe you would have to be pulling around 14000 lbs to have the benefit of the 4.10 gears. The truck he found just happen to have the 4.10 gears. Saying all of that I agree with JWB123 that for towing you can't beat a truck like your diesel.