When it comes to individual campsite reservations, there is no difference between a vigilant
travel agent and a vigilant camping guest. The method used to secure a reservation is basically the same for either one.
The TA may have better "luck" because as a matter of survival/necessity, they try harder to secure the early reservation when the booking window opens AND, if they don't get one at opening, usually try OVER and OVER to get one if a cancellation/opening becomes available.
It's not a matter of "luck". It's a matter of persistence. I got a Halloween reservation a few years back because I was online booking at the minute the reservations for that arrival day (in late October) opened up. TA's have no "magic", no "back door access" for individual sites. Most just work hard, wake up early, and do the best they can for their clients. Groups for TA's are a separate, contract issue but for the onesie-twosie campsites, it's ever "man" for themselves.
In my opinion (of course)

. Becuz WTH do I know, right?
Bama Ed
PS-Of course, I have no idea. None whatsoever.