Maple Grove Resort seems to be the best for the Hwy 12 area. It's a membership park - RPI/Coast to Coast but if not a member you'd just pay a higher price.
If you do any searching for this name, there are two named the same in Washington so make sure you're reading about the one near St. Helens/Mt. Rainier.
We stayed at Iron Creek national forest campground down the road.
Both are an easy drive to Mt. St. Helen's Viewpoint on the east side of the volcano off Hwy 12. It's a beautiful view.
The other viewpoint from the west side is accessed via I-5.