I'm fixing to go to the new 6.4 Hemi in a Dodge 3500. Just haven't decided on SRW or DRW is about the only holdup on ordering. From research and going with 3.73 gears, expecting to average about 16 mixed empty driving and 11 average with a 4,000# camper, maybe better. Truck Camper Magazine reported 12.8 avg with their rig though believe that was a lot of highway driving.
The new 6.4 is designed to be a true truck engine with most torque available below 3000 rpms so no need to red-line the engine to get the power. I test drove on for over 40 miles and was extremely pleased with it's performance.