My 2c. Don't focus on fuel usage. Recently we went from a Prado TD to a Prado V6 petrol. Towing with the TD we got around 16lphk. Towing with the petrol we average around 22.
But the difference in the two is night and day. WIth the petrol overtake when you want to overtake. When you are not towing the difference in economy with Prados in petrol and diesel is not that marked. On the highway the D4D will get around the 10lphk, our V6 regularly gets 11.6.
And, the big one, no diesel carbon issues. I love diesel, did my trade on them, but there is a reason they get reasonable performance out of them nowadays. The electronics ring every last bit out of them and reliability is falling. The petrol in the prado is one of the sweetest around.
After investing 50K on a van, and 60k on the tug, fuel is not really a big issue. Just my thoughts.