I did Route 66 last summer in 3 weeks
Was organised by Virgin Holidays and included the flights to Chicago and home from LAX. Accomodation and bog standard car were also included
I take the point made about it slightly restricting where you have to be on any given night but they did try to give more time in the better spots - Chicago, Vegas, St Louis, Santa Barbara etc. and just a night in the smaller places
We actually took an extra night in Vegas come to think of it instaed of Flagstaff I think it was. Vegas hotels were dirt cheap anyway
Its a great trip you'll have a blast