Custer State Park - look around the Blue Bell Campground area. You have the badlands, Mt Rushmore, Needles Hwy, Spearfish, awesome hiking, swimming, and really nice campgrounds. It's about halfway - 18-20 hours from Michigan.
I don't know what it is from San Diego, but looks to be about halfway.
We really enjoyed our 2 weeks there last year. Weather is best in Late-June to Mid-July.
2014 Forest River Shamrock 21DK
2012 Ford Expedition Limited
Load Leveler / Sway Bar (800#)
Tekonsha P3 Prodigy Brake Controller
2016 - Four trips, 11 states, 19 nights
2015 - Four trips, 12 states, 17 nights