In my travels, the state parks have their own sites and some number do their own booking, some will use a national pay site like reserveamerica. Last year I ran into a catch 22 where I went to a Kansas park that advertised first come first serve but the only way to pay for it was online (ranger station closed) and there was no cell service from the campsites. Generally though it's navigable, really no worse than reserving at RV parks that have their own booking sites. Accommodations were all pretty good, even as random picks. Some of the best spots were COE parks, easily booked ahead of time and with the senior pass were often half price, sometimes less than $20 a night with electric. We leave the majority of our trips unreserved for maximum flexibility and have yet to be stuck without a place to stay. There's always cracker barrel, bass pro shop, wally world, etc if all else fails. I use a garmin RV gps that can do area searches at my destination so the day before I'll scope out what's around, we always end up somewhere...
Mark B.
Albuquerque, NM