OP, you'll get lots of opinions here from those that almost exclusively overnight in free spots and those that only drive 4 hours a day and then relax for 16 hours and those that are too good to stay in a parking lot and those who generally don't sweat the small stuff and may or may not free overnight.
I believe I'm in the last category. We also may drive 6 hours or 18 hours depending on the trip. (We are not retired)
Bottom line, while traveling we rarely make reservations, unless over a holiday weekend or a known popular area. Destinations may or may not be reserved but considering most of our camping is in the woods somewhere, alot is boondocking or first come first serve.
You can stop any number of places just for a sleep, but if you're doing the retireee program of only crossing 2 counties before stopping for a night, then don't be cousin eddy in the Crapper Barrel parking lot!