I owned my 07,5' DPF equipped Chevy 3500 CC/LB/D/A in LTZ trim from July of 07 thru July of 2010 and I never had the first issue with it. I used it to tow my 15,500# 5ER through the mountains of SC, NC, TN, VA and WV with again, zero issues... Don't remember taking that big of a hit on MPG between The 05' LLY Dually I owned and the 07.5'...THOUGH, you DO get better MPG with DEF equipped trucks, and when the 07.5 went into regen...you saw a drop in MPG then...a lot depended on how one drove their truck..
As was stated, most aren't really thinking about MPG when towing heavy with a 1 ton, crew cab, long bed, 4 wheel drive dually...they just need it if they're going to tow a certain weight 5er.
Others are correct, The 07' Classic didn't have DPF and the 2010 still had DPF, but a little more refined and DEF, which helped with better MPG.