If you're talking about new full size vans, the selection of those with significant towing capacity have dwindled since years past. You can no longer buy a diesel full size van with a towing capacity much over 5000 lbs. GM no longer uses the V8 Duramax pickup engine in the Express and now offers the little 2.8 liter diesel used in the Colorado. They do offer the 6.0 V8 which has about a 10,000 pound tow rating. Ford discontinued the Econoline van and now offers the Transit with a range of engine options, but none of them can tow like the Econoline. Mercedes has the Sprinter with a diesel, with towing ability similar to the Transit. Nissan has the NV series with a gas V8 that can tow pretty well. Ram only has the Promaster, which has to be converted to a passenger configuration aftermarket if I'm not mistaken. It's also front wheel drive which makes it pretty unsuitable for heavy towing.