When building a tow vehicle that will serve as a daily driver, it better to build it for the rule, and cover the exception. Fact is, most tow vehicles spend far more time without a trailer attached to their asses, than with. The truck should be set up to handle that. But, since most of the time it's being used as a car, why overbuild or over buy?
The suggestion for a built 318 is sound, in that, for the most part, the truck will give decent fuel mileage for 90% of it's use. With proper gearing, and breathing, a 318 can easily serve both applications, without the expense of a new diesel, or the ultra poor mileage of a V10.
As I stated earlier, you will not be the fastest rig on the interstate, but you will be able to maintain the speed limit +. Besides, I usually try to save the honor of being the fastest on the road for some other idiot. Being the fastest on the road usually results in being issued a citation, awarded by the state police.
The decision to put a Hemi in the car being towed, not withstanding.