Going from a 5.3L Tahoe towing a 5500 lb/24' trailer to a 6.6 Duramax GMC Crew cab 2500HD towing the same trailer I went from 9.2 mpg average to 12.6 mpg average over the same set of long trips. I also picked up way more power... but I'm guessing you'll see similar MPG gains. The diesel will have a flatter torque curve and less feeling of drama for normal driving because it probably won't need to downshift so much. It may be a bit slower, but you'll probably like the driving experience better. Diesel and Mid-Grade unleaded have been running about the same price on the west coast lately.
I use my GMC as my daily driver and my only real complaint about it is that it's huge. It's super comfortable, gets about 15 MPG (diesel) and 1500mpg (DEF) in mixed driving, and it's been quite reliable (I have 83,000 miles on it). The 3/4 ton suspension I find to be comfortable enough in unloaded driving, and it really makes the trailer towing nice because it doesn't porpoise. The other really nice thing about the truck is the exhaust brake, which I'm not sure the baby diesel has.