I've also stayed there 2 times (once last July 4th weekend) and have made my reservations online. Follow the website link on the Campendium page, scroll down and you'll see the reservation area. I liked the idea of having a reservation in hand just in case there was an issue although both times I stayed there was plenty of parking. As said below, turn right after passing through the guard gate. That's where RV's have always parked when I stayed.

Ive stayed there a couple of times. There's no reservation system. Just drive up to the gate house and tell them you want park/stay in the lot. This is a semi truck parking lot.....it's huge. Once you enter turn right and go wayyyyyy out there where there's no trucks. The trucks always come and go, some idle their engines for hours. If you find a distant spot you won't hear them. There is nothing here but a cement parking lot. Nothing, no water no electricity. The only thing good about it is it's closer to downtown. I stayed about three nights each time, since it's the only place there is in Chicago.