It can be very difficult to find a place to camp during summer weekends. For anyone that regularly climbs at Devil's Lake, you should consider joining the Chicago Mountaineering Club. The $25 annual membership (+$15 campground fee) allows you unlimited use of their private campground which is located in the park.
If you want to climb, try to get one of the group sites. Most are for 20 people and there is one 40 person site. These sites are at the base of the cliffs, so you don't have to drive anywhere the morning of you climb.