I don't mind you guys making fun of me because of that, I can stand the heat ;-)
But no, mpg is not a main concern, like you said, a PW will never be good at that. Main concern is pavement traction in dry/wet conditions (which a highway tire will always do better than an offroad tire due to a different channeling structure and less negative profile) and road noise. The PW I test drove with the Duratracs had an audible hum even at 30 mph, so it's probably only going to get louder at 70 mph.
Just looked at the Nitto Dura Grappler, the 285/75/17 (34") weighs 67 lbs compared to 50 on the Defender in 285/70/17 (33"). That sounds like a lot regarding unsprung weight, but I don't know what the rims weigh and if it really matters on a 7000lbs truck...