Can I chime in? :) I went from a 5.9L Cummins to a 6.4L Hemi.
I don't mind the extra RPM as I know that's where the power is. When I climb a steep grade with a GCW of 21,000#, it's going to hit high RPM.
MPG wise, I have pulled the same 37' 5er with both the 5.9L Cummins and the Hemi over the same terrain. I lost 2-3 mpg towing, and 2-3 mpg running as a daily driver. The diesel would get 10-11 mpg towing the RV and the Hemi gets 7-8 mpg.
Where I live, diesel is $1.369/L and reg unleaded gas was $0.999/L when I filled up yesterday.
I have no regrets.