A better place to camp for free is a small lake just over the Idaho boarder and only few miles from Marshall lake, called Freeman lake. It is smaller then Marshall and Freeman is more of a Bass and spiney ray lake. It also has Tiger Muskies and Pike in it in small numbers. It is planted with Rainbows as well. Since they are so close you could do both lakes.
The campground is primitive with no hookups or drinking water, but as far as I know it still has an outhouse. Haven't been up to it in a few years. The West and North ends of the lake are good Bass fishing as well as some of the south end. If you are there on a week day or on a weekend that is not one of the prime camping weekend you will be most likely the only person there.
Here is a look at the lake and campground area;
http://maps.google.com/?ll=48.223186,-1 ... 8&t=h&z=17
As you can see there are no houses on the Lake and only a couple you can see from the lake. It is surrounded on 3 sides by mountains making it a very pretty spot to camp and fish. The bass fishing can be quite good on the west and north end of the lake along with some of the south end. I've gotten L.M. bass up to 5 lb. from this lake. With a fly rod you can get any number of bass, trout ,crappie, and sunfish on a dry fly in the eveing. Here are the best spots to fish for bass. You can get a one day license in Newport for about 12 bucks.
Here is turn by turn directions on how to get there from Newport Wa.
http://maps.google.com/maps?saddr=Newpo ... s&t=h&z=13
Inventor of the Fish-N-Fool Knot , Winner of Knot Wars
Watch my Fish-N-Fool Show on YouTube