the usa has lame cities compared to europe, in the states it best to plan nice streches of roadway between note worthy landscapes. example between sanfran and portland dont take the i5, take coastal 101. grand canyon? dont use the i40 try historic parts of the 66 and 89.
i would not buy a car, just rent something that you can fold down rear seats and sleep in. so you dont have to get a motel every night. also the older senic hiways have cheaper older motels if your on a budgette - save your money to splurge on a room in vegas.
try to get at least 6 weeks, there is not real much here in the states, however traffic and huge spaces between sites will eat up large amounts of time. i allways felt 6week to 3 months was a good journey, every other over seas excursion less than that felt like of a waist.