Maybe not as secluded as you wanted, but just north of Dallas is Bonham state park. Its about an hour or so north of Plano. It is a state park, but they have the primative camping and its usualy a lot less crowded then the other state parks. They have a great little lake where you can rent a canoe and stuff. Also, the surronding area is all great small towns.

Anyway just thought Id throw in my two cents worth.