Not sure about that size, but for our Chevy there is still a limited tire choice in 20" E rated tires (265/60/20). Before buying, do some searching for replacements if there is a certain tire model or type you prefer to go with when the time comes.
As for towing, not sure I have noticed much of a difference at all, at least compared to our '10-1500 that had 18's.
The 20's do ride rougher on the roads here in Mi as the majority of them are ****!!!! On a nice, smooth roads hardly any difference though compared to the 1500 w/ C rated or even the stock tires.
I know the Rams do have a little room in the wheel well that allow more tires sizes to fit compared to our generation Chevy if you choose to change the tire size.