Using your numbers this is what I come up with
Diesel HD truck:
Purchase price: $50,000
Fuel cost over 100k(diesel at $3.00 and 16 mpg): $18750
DEF fluid over 100k (at $5.00 and 800 mpg):$625
Resale 5 years later:$31,000
Total cost: $38375
Gas HD Trucks:
Purchase price: $42,000
Fuel cost over...