I towed a 12k 5er with mine. It had 3 slides and was 37' long. I was very impressed with how it performed, and I had 2 Cummins (no "G") diesels before I went to the Hemi. In fact, I towed the same 5er with a chipped 5.9L Cummins and with the 6.4L Hemi. On the flats, the Hemi was at about 2200 RPM in 5th at 21k GCW. Into a wind, it would use 4th at about 2800 RPM. In the hills, it would use a lower gear and rev at 5000 RPM at times, just like it was designed to do. I got 7-9 mpg pulling my 5er where the 5.9L Cummins got 9-11 mpg with the same 5er and rarely downshifted - it took a small hill or decent grade to cause a downshift.
The Hemi would outperform a Cummins of the same year (370hp/800tq) with 3.42s on 0-60 when towing 14k. My friend has a 40' 5er that weighs 14k and we tested it.
The Hemi was able to accelerate (albeit at high engine RPM) whenever I wanted to when pulling my 12k 5er. It did use high RPM and use more fuel to get it done. Engine and trans temps etc were never elevated, even when working hard with the EVIC set to display them in numerical digits, not just a needle pointing at "Normal".
I'd buy another in a heartbeat.
If you tow a lot, a diesel is a better choice, and will make for a better towing experience, but I was perfectly happy with the 6.4L Hemi I had. It will be even better now that it's paired with the 8 speed auto. The 1-2 gap in the 66RFE was always a slight bog down spot with my truck. Mine had 4.10s, and I read this flat spot in the acceleration is more pronounced when the truck has 3.73s.