Just one thing that has occurred to me. When I check distances between places on Google Maps it gives me a "Driving Time" as well as the mileage and if you do the math, mostly this works out at an average driving speed of over 60 MPH ! As I understand it, the USA has a national speed limit of 55 MPH and I would reckon that on standard highways - like 1-15 and 66 - it would be difficult to average more than say 40 MPH whilst actually driving and with the inevitable stops to take photos, coffee and stop at attractions, I'm guessing that 150 - 200 miles a day is the maximum one should allow to make it a holiday rather than a chore. Would this be a good target to aim at.
Richard