I tow a 27 foot (hitch to bumper), 5500lb travel trailer with mine, towed it with my 17 and my 19 also. I would say the 21 is a better tow experience but it still sags pretty bad without the Equalizing hitch, I use the R6 with 1200lb bars and they keep it almost level. I'd say it isn't optimal with my setup but I don't have any problems either. Not to sure what my tongue weight is but I'd guess about 550 lbs or so. I don't use the backup assist or the sway control as the R6 sway control seems to work fine. I have the 37 version and it has plenty of power, gets about 9mpg on average at freeway speeds with no wind. If I am on back highways at 65 I get about 10.5mpg. In Utah at 85mph (speed limit there was 80) I was getting 6mpg. So overall not to bad for a truck that really wasn't designed to tow.
Best thing is the 37 gallon tank, With my other trucks they were the short versions and the 22 gallon tank was a bummer while towing.
Not sure if this helps but its my experience.