I take secondary roads all the time but they are quite a bit slower. All those stop lights and 30 mph zones after all. We'd do 500-600 miles on the Interstate and maybe 300-350 on secondary roads
We have all summer to travel now I'm retired and will have all year in another year when my wife does. But many folks don't and likewise we didn't have the money to rent a Class C near our destination we needed to travel on the cheap with two or three kids along.
So it's Interstates for the first part of the trip. I agree about I-70 in the Midwest that why I suggested I-40. Maybe not the most scenic but better than Illinois.