At Exit 252 is Hilgard Junction State Park, right next to I84. Pay park, some road noise, but right on the Grande Ronde River.
Just before that (going east!), if you get off at Exit 248, is Forest Road 21. About 2-4 miles on that road are a fair amount of dispersed campsites easily accessed. There is also a small, no charge, Nat Forest campground, Spring Creek. Small, but you could get in there easily. Next to (!!) Spring Creek!! One item of note, is that this area is also used by a lot of ATV enthusiasts, esp on weekends, and could be noisy.
Near Ontario, is Farewell Bend State Park, nice, clean, sometimes very busy!! Again pay stop. Never have seen them, but there are supposed be some boondocking spots along the Snake River in the area of Exit 356 along hwy 201. Just dirt or gravel spots next to river. Can be see on Google Earth.
Those are the ones I'm familiar with along your route. As to Lolo along Hwy 12, there are a few non NF areas, not many, and you have to be real aware of them or you'll fly right by with a poor chance of being able to turn around!! There are some very nice NF campgrounds along the hwy though.
Rereading your orig post, I see that you will be heading west, so just reverse my Oregon spots itinerary-wise!!!
