Well... to keep you off interstates and many nice public parks along the way (national forest/state parks) here's a route we like.... 1,000 miles and because it's secondary highways it won't be fast driving but worth it! I'd plan no more than 250 mi./day if that. Pull into the campground in the afternoon and relax and enjoy the scenery.
Bandon, OR on 101.. go north to Reedsport and pick up 38 to I-5.
I-5 a short distance north to Eugene to 20/26 to Sisters, OR
26 east (John Day Fossil Beds/Clyde Holliday State Rec. campground)
26 to 201 at I-84 Ontario, OR
Pick up 201North to 95 at Weiser, ID to Grangeville, ID.
Hwy 13 to Kooskia, ID to ID12 - a nice drive along the river to 93 to Missoula, Montana
to 35 along Flathead Lake to Columbia Falls, MT at Glacier Nat'l Park
Here's a good site to find public campgrounds: