What you are looking for, as per your description, can be found a little further North and East. Canyon Ferry reservoir (also Hauser and Holter reservoirs), North of Townsend is just such a place. Bureau of Rec owns it. Free camping lakeside, at Goose Bay (east side, and my absolute favorite)......White Earth and Silos on the west side (these two sites are a nominal nightly fee). There are also nominal fee CGs along the Missouri river reservoirs all the way north of Helena. You would easily fulfill all the requirements you seek. Fishing is very good for trout and walleye. Plus, if you've never been down "The Gates of the Mountains"......you be missing a very beautiful boat ride, not to mention the scenery (and did I mention the fishing?). I take EVERY person that comes to visit us and likes to fish, or just plain enjoy a boat ride there.
You can PM me for more info if interested.