I'm not sure what the premium they are putting on diesels in the vans but with the pickups there is basically no way the fuel savings will ever cover the up front cost of the diesel unless you are driving commercially (ie: massive milage).
With the pickups, you can justify it based on towing a heavy load but with a class B, that is unlikely to be a justification.
It's nothing for modern gas engines to reach 250k miles so the longevity of diesels is no longer an issue.
The spreadsheet has it about right. For 120k miles, you may save $4k in fuel. If that is less it may make sense but diesel repairs are also somewhat higher and again we aren't in the old days. They need repairs about as often.