For me this has changed as cell phone communication and internet search has become common. Back 17 years ago when I began RV'ing I would buy an annual camping guide with a list of subscribing RV parks. I would call two or three days in advance to assure I had a place to stay on a specific date. Today I will only reserve if I intend to stay for an extended period. Sometimes I get "burned" but normally I will not begin searching until mid afternoon on the day I am traveling.
After more than 150 thousand miles of trailer towing, I've learned not to sweat the small stuff.