I am not aware of any major RV manufacturer who built on the Chevy Express van chassis with a Duramax diesel (although some built using the earlier 6.5 Detroit, pre-Express Chevy Van). The price add-on makes the diesel a hard sell, and the extra weight limits how much house can be built on the chassis. There have been smaller manufacturers who have built to order, price no object, but more often they would build on the E-series with the 7.5 diesel, then later the 6.0 Powerstroke.
Larger C's on Kodiak or F-550 sometimes have had the diesel as an option, and there are some small builders who used the Dodge 3500 cab-chassis with Cummins. However, many of these larger C's also had gas engines (Vortec 8100, Triton 6.8 V-10) because of the same cost and weight issues. You will also find builds on International and Freightliner MDT chassis, always with diesels. Manufacturers in this class include Dynamax, Gulfstream, Thor and Jayco, from about 2003-4 until the market crash. Dynamax failed and got bought up, but I think the brand still exists; Thor has gotten back into this part of the business since 2010, but no Chevy chassis because GM once again exited the MDT market per terms of the Federal bailout.
Tom Test
Itasca Spirit 29B