You can use my property on hwy 504 at milepost 19.3. It may be too far from where you are, but you can park trailers or whatever and pitch tents and have privacy. There is also an electrical hook-up and water. Close by is a restroon facility. Also I can provide firewood and some picnic tables. This is located near the mouth of the Green River and across the road from the N. Toutle and about 40 miles from Mt. St. Helens. There would be no charge.
If there is anything else you might need, let me know and I might be able to help.