We started as junior road warriors back in the 1950s traveling with the family in a 1957 Mercury Turnpike Cruiser. Dad never made any arrangements ahead of time. He trained us.
If you avoid the peak times to visit the most poplular places, most of your troubles will be over. Good to start and end early. Good to call ahead for a reservation on the same day. Once in awhile you can get stuck. There are always alternatives. I could never go on a long trip with all the places to stay mapped out. Same thing traveling out of the country. My brother has been everywhere and traveled for years around the world. He is in the Phillipines right now. His motto "the worst that can happen is that you sleep in your rental car and have rice and tea for dinner."