Interesting that you ask this question at this time. Three young men recently completed a trip to all 48 states in the lower USA and set a new record for the fastest time. They drove slightly over 7000 miles (nearly 12,000 km) in 106 hours. They followed a circuitous route that managed to touch all 48 states.
That is slightly more than four days. Naturally, they drove day and night, stopping only briefly for fuel, food and toilet breaks. They did not have much chance to see many of the attractions. They did set a record for the fastest driving time to visit all 48 lower states.
Here is a link to their web page. It shows you their route. Web Page