I live in Kansas and they are NOT free. For most with electric and water it's around $16 or so. Milford Lake, Cheney Lake, and El Dorado are all nice just depends on where your going and what your doing. I wish they were free though!
It may have changed, but there use to be a difference between Kansas State Parks and Kansas State Fishing Lakes. Here is a link to one of the State Fishing Lakes http://baldwin-city.com/rec/douglaslake/ which states:
"Camping at Douglas State Fishing Lake
Camping is allowed on the area and is free to the public. There are no electrical hookups or shower facilities. Three pit restrooms are available."
The website does not say when it was last updated so I would recommend checking with the Kansas Department of Wildlife and Parks, Clinton Unit, RR 1 Box 120E, Lawrence, Kansas 66044 ~ (785) 842-8562
Regional Office ~ (785) 273-6740
In the past Kansas also allowed camping on some of their Wildlife Management Areas, usually in designated sites that had a portapotty and maybe a picnic table or two. It was always during hunting season when we stayed there so I don't know if camping is permitted all year.