The "higher value equation" has already been established and it's $3000. Not much for a vehicle 3 years newer.
So in another 5 years, the bare bones diesel might still be worth $3000 more, might not. The fact it was a rental, not knowing how well it was maintained or driven, the $3000 add'l price could increase substantially.
Also, not a lot of people may be interested in a bare bones, contractor style XL. That will also dictate it's value.
Take a drive through a Uhaul lot. Don't see too many diesels anymore.
S