I am about to get myself into a big commute situation. I have a 2800 mile round trip to make once per month. Right now I have some great cars but nothing that I want to put 60k miles a year on, or any that get better than 20 mpg. 400 miles each way is going to be through Colorado so I think a turbo model is in order unless I want to become a rolling roadblock

I would also prefer to spend $15k or less as the miles I put on will kill the resale anyway.
I am a pretty big guy so an S-class is tempting. What would you choose and why? What kind of milage can I expect cruising at 80-85 across Nebraska? Thanks for the input guys!
PS, I hate commercial air travel