I don't think this Nissan/ Cummins half ton is going to be that great. This is the first I've ever heard of Cummins making a V8.. ever.
All your math and reasoning is correct but what gasser gets 20 mpg? I'm getting 9 mpg in my 1500. A few of my buddies in their diesels get around 25. Diesel fuel may cost more but at over double (more like 2.5x) better mpg diesel is way cheaper to run than gas.