This is complex because of US emissions law. The big three use truck engines in their vans. This is not ideal because of the large displacement, unnecessary unless towing heavy trailers. The import options are technically ideal, but parts and service can be an issue. I would choose the Mercedes sprinter, parts are expensive but available, rarely needed. A 2006 or older Duramax from GMC can achieve 20 mpg and offer incredible durability. I’d avoid Cummins or the Ford Navistar engines unless you’re considering a 7.3 or a 12V Cummins.