Actually, check-in times vary by campground, and sometimes by season within the same campground (tends to be later in high summer). It depends on several factors, including whether the check-in desk is manned by a Corps employee or a volunteer, whether the sites are improved or primitive, and whether the campground takes reservations or is first-come-first served. My best advice is to visit the campground's web page and find out when the check-in time is, and plan your trip so that you don't arrive any sooner.
fltlevel510 may have arrived early and set up with no problem because the check-in desk was unmanned, but don't count on it happening that way all the time. As often as not, if you arrive early and set up right away you'l be charged for an extra day.
www.CorpsLakes.us - Corps Lakes Gateway - To select a Corps campground, start here. You can search— by state, or by lake, or by activity type— to find just the right campground to suit your tastes.
Note: As a courtesy, the Gateway also lists state parks that are located at Corps project lakes. One drawback to this site is that they don't usually provide links directly to each campground's web page. Easiest way to find that is to Google the exact name of the campground you want after selecting it at the Gateway.
To go to the campground's web page, go to the reservations page at
http://www.recreation.gov/ - You can make reservations through this site for many Corps campgrounds. However, as noted above, some Corps campgrounds, especially those with primitive sites, are first-come-first-served.