Sales brochure specs do not list the specific tire size that was used.
Its possible that there is a placard somewhere that lists the weights and other chassis information, that has the tire size on it. This would have been a GM installed or provided placard.
The 215/85R-16 is what the Sprinter with duals uses. Just make sure that the max tire weight adds up to the GAWR (gross axle weight rating) for the rear axle. Be sure and use the lower weight rating for dual installations. If they don't look right, they probably are not.