They are not in the sense a state park designed for recreational camping. Each state has a big state fair grounds AND all little local towns usually have their own fairgrounds facility.
Fair grounds are used all year long for events and the 'exhibitors' and 'vendors' have LQ's, TT's or MH's so that is why there are hookups for RV's at fair grounds. I used to show horses which is how I know where to find the dump stations at the grounds that don't have full hookups.
Best one I stayed at was at the Iowa State Fairgrounds. Over a 1,000 CG sites with full hookups.....I was the only one parked there in the off season! :C
Worst one was Florida's State Fair. Don't stay there 'during' the fair! They don't require sewer donuts and whew did it smell.