I am unsure of your question about DEF in the 2015 Chev's. You say you have a 2011 and they use DEF...I have one. It was the first year using DEF for Chevrolet. As far a dually, I would never go back to a single wheel truck pulling large fifth wheels. I had a blowout on our 2003 chev duramax pulling a 34 ft Alpenlite and it was a bit squirrely. Lots of weight on those back two tires. I have noticed that the tires on our 2011 dually have lasted quite a bit longer than when pulling with a single wheel pickup. The stability makes trips with winding roads a lot more stress free. Best of luck...