Nothing ruins a trip faster than needing to be in a certain place at certain time. Having said that, we make reservations when need want to be somewhere (relatives and friends over the holidays for instance). On our winter trip, we only made advanced (more than 2 weeks ahead) reservations twice. The rest we just looked at which way we were going, set a reasonable driving time and went to RV Park Reviews or allstays to find a park.
Those places that fill up over popular weekends we tend to avoid because of the crowds. Plus, we usually move mid week.
I might add that if all the new RVers that the industry says are there all go camping on the same weekend, the Wal*Mart parking lots will be full.
In Lafeyette, do not miss Poche's Fish and Camp in Breaux Bridge or the market with the same name.