There's a 5.7 in my '04 Ram 1500 & I've never gotten better than 10 or 11 mpg while towing either an open 16' car trailer with a car, or a 24' camping trailer at 6500 lb.

I've gotten 17 mpg hwy by itself at 65 to 70 mph, & less than 10 mpg city.

I think the newer Hemis will be a little better, but I doubt it'll be drastic enough to matter much.