Mercedes can claim 37mpg for their diesel, but can they deliver? Nissan has access to both Renault and Mercedes diesels, but they are working with Cummins on the Atlas 4 cylinder. The rumors are hot and heavy the Atlas in combination with the ZF 8 speed will show up in the next generation Titan and van.
I don't know how different the Saltillo built vans are from the Euro ones. The Ducato offers a very sophisticated CNG version overseas. Unlike the Ram CNG pickup, the Ducato's tanks don't eat up cargo space. Money is tight at the moment so that might push off a CNG fueled version of the Pentastar. That would offer savings over the CNG 3 liter four cylinder currently offered. The 3 liter doesn't fit in the Scudo, while the Scudo replacement will offer the Pentastar.
The Doblo is offered with a bi-fuel option (gasoline or CNG), which is the forerunner of the Ram pickup CNG system. The engineering work has been done, but who knows what we'll get here when it comes.
The current Ford and Chevy vans offer factory approved aftermarket LPG and CNG conversions. I don't know what the prices are, but I suspect they aren't as pricey as the Sprinter. I bring this up because the fuel prices are vey attractive.
The current rage in pickups is gasoline fueled six cylinders. Both the upcoming Ford and Dodge Vans have this standard. Mercedes isn't offering a gas motor in the US.