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Port St. Lucie
From Memphis to Council Bluffs, Iowa. From Council Bluffs to Rapid City, South Dakota.
I would spend the next day at Mount Rushmore, Crazy Horse Memorial and Custer State Park Needles Highway.
Continue on I-90 W to Seattle. It is an 18 hour drive from Rapid City. I would drive 6 hours a day for the last 3 days stopping at your whim.