Jos, I understand your wants, and I sympathize with them. But I think you're mistaken by thinking that buses only run between big cities. Obviously they do that as well, but if you want to see "Real" middle America, there's no better way to do it(imo) than on a 35 year old Trailways or Greyhound bus pulling into Apple Pie, Oklahoma and leaving for Mom's Kitchen, South Dakota. The people riding those buses between small towns are the locals whom you profess to wanting to talk to.
But I do understand your desire to do this as self sufficiently as possible. It's unfortunate I don't have anything to offer you by way of assistance other than a sofa to crash on if you make it to Sacramento. Best of luck to you. I hope you have a great trip.