I suggest you look into LBJ National Grassland north of Decatur or Caddo National Grassland northeast of Bonham. You can find more info here,
http://www.fs.fed.us/r8/texas/recreation/index.shtml
Don't be fooled by the name "grassland". There are lots of trees and hills and several small lakes. We have never camped at either of these areas, but they are among our favorite places to hike. If you are in the DFW area, both of these (and especially LBJ grasslands) would be a lot closer than the National Forests near Huntsville.