I have a 2011 Silverado 3500 HD with the Duramax. It gets about 12-13 mpg when towing, and about 16 when not. I don't know if that's "best", but it's good enough for me. And it's for sale with 12,800 miles on it, and still under factory warranty.
Depends on what you want. If you want the max amount of money and are willing to wade through the scammers and to deal with people directly, go through Craigslist or RVTrader. If you just want to get it off of your hands, sell it to a dealer or go consignment. You'll take home less, but you'll have no hassle.
When I got rid of our MH last fall, I went the consignment route. I figure I lost about $5000 on the deal, but it was worth it not to have to deal with people coming by and worrying about getting ripped off.
I only had one guy come by to look at it, and he bought it.