I too just bought a new 2011 LML POWERED HD3500 dually. Never owned a diesel powered anything before. I have no intentions on modding anything on this truck. I was amazed at how expensive the filters are for the fuel system.
Thats great to hear about the towing milage. I just bought a 2011 3500HD dually to tow our 10K fifth wheel. Never owned a diesel truck before. Got 17 MPG on the highway, not towing of course. So far so good.
Had a 2010 2500 6.0 gasser towing a 10K fith wheel, was getting 8.5 MPG. It pulled the trailer just fine. Just bought a new 2011 3500HD dually to replace it, haven't got to try it yet