I have stayed at municipal and county fairgrounds in Kansas, Oklahoma, Missouri and have visited the camping areas of several more, if you want to broaden that to include rodeo venues, horse racing tracks and regional exposition centers.
Facilities vary from a tight parking spot with water and electricity to full service RV parks, and I've paid as little as $2 a night, as much as $40 in a fairgrounds RV park. Most I've seen don't take reservations for non-event periods, but some of the more upscale RV parks at fairgrounds do.
Fairgrounds etc are an option for me only when there is no major event going on. The RV parking areas are usually way undersized for demand during a fair or other event, and while some places have spill-over areas with lesser facilities, those get filled too when something is going on.