We don't like to make reservations, at least weeks/days in advance as we never know exactly where we'll be when. With that said we do subscribe to the WDOS (WeeDOS) system, and that being Week Days Off Season. These days are generally fine to get a spot for a night or two, but no long term. Weekends are another animal altogether. We generally try to make a reservation for both Friday & Saturday, and if necessary the same park. (We move a lot). Sunday is generally fine and back to WDOS. We found that this is generally true with the exceptions of popular national parks, Yosemite, Yellow Stone, Smokey Mts, etc.
We generally avoid private parks unless necessary.