We never had reservations over a day ahead during our cross country in 2014. Usually, DW would call ahead the day of our arrival to secure a site.
The biggest reason for this was our route and travel times were always subject to change. I didn't want to be tied to being in a particular place at such and such a date.
The only time we called ahead a week in advance was when we were staying in Los Angeles. There is a state owned, LA county run RV park right on the beach in LA called Dockweiler. This is one of two RV parks in LA that we made advance reservations. The other was Balboa RV Park in Van Nuys.
Otherwise, it was the day of arrival when we called ahead to make reservations.