After October and before April you can just pull in and pick any spot. More than likely, the DNR office will be closed anyway. After you get parked, eventually one of the DNR folks will come by your camper to collect from you. They will be driving a vehicle clearly marked with the DNR logo and they will be wearing a DNR uniform. So no mistaking them when they come knocking on your door.
You can also go out to the DNR camping web site and punch in your State Park and go to the Camping Reservation site. There you will be able to pull up a map of the campground, select your campsite, look at a couple photos of what it actually looks like, and it will give you the length restrictions and how far water and electric hook up is.
No this time of year you cannot make a reservation date before April 1, BUT you can see the site on the reservation web site. Meanwhile, just pull in. We've been to every State Park and State Campground and Recreation area campground in Indiana. We were able to get "reservations" for multiple sites that easily accommodate a 40 footer. In March, you will probably be the only camper there, you'll have your pick-of-the-litter.
Edit: One other thing you need to remember, the water is shut off also. And the bathrooms are closed up and locked. You will have access to open pit toilets, and you'll need to supply your own water. You can use the dump station, but you won't be able to rinse your hoses, unless you do it with your own water. Those faucets will be turned off also.
