Rifle Gap has 5 campgrounds and 89 sites and they are nice. You just need to go on their web page and see what spot, campground you want. Prices range from $22 to $38 a night. Per the web site "You can reserve your favorite campsites up to 6 months in advance! Reservations have to be made a minimum of three days prior to your arrival. All unreserved sites can be occupied on a walk in basis if it is not already occupied."
Can fill up fast so reservations are probably a good idea.