Getting RV sites on the road

I make reservations for major stops like Yellowstone, Glacier etc and I usually same-day-called ahead (tried to call about 2-3pm) for my travel days in between. We spent a few nights in Walmarts and truck stops, but that was our choice in order to make mileage, (except for once) and not have to stop early just to end up in a particular town. I was travelling by myself with two tweens and every few days of hard driving mileage runs I would spring for a KOA and early stop/late leave to let them swim and goof around. We never got off early in the mornings without lots of drama, but they would tolerate driving late into the day, so this pattern worked for us. I used several apps All Stays, Good Sam, KOA etc.