They do have RV spots, and from what I can tell they are really slow this time of year, so you should be able to get in the day of, however they do have a reservation option too.
https://www.accessarkansasparks.com/...esResDesk?6027
That should get you to the site for Lake Fort Smith spots.
Just FYI I am going to be setting my camper up in one of the spots at the top closest to the visitor center, bc it will be right by one of the holes, and I have a dg friend who is going to stay in it.....you may want to get up there too, or that may tell you to go to the other end!
I can't remember the site number, but will find out tomorrow when I am there.