Kinda depends on what you're looking for:
We took Hwy 2 out of Spokane over to Grand Coolee Dam, then south to Soap Lake and picked up 90 to Seattle. Tour Grand Coolee Dam and the Dry Falls area. Several State Campgrounds along that route suitable for big rigs. We also stayed at Soap Lake and enjoyed the theraputic effects of the lake. Was the choice of movie stars in days past. Moses Lake is upscale if that's your thing. You can pick up Interstate 90 there. If you head over to Seattle, stop by Snoqualmie Falls. Not to be missed. Out of Seattle, head south on Interstate 5 to Lexington. Turn West across the Lexington bridge to Washington 411 and head south. You'll meet up with Washington 433 just north of the Lewis and Clark Bridge. An awesome bridge, cross the bridge and turn right onto US 30. Stop at the top of the hill for a Kodak Moment overlooking the bridge. Take 30 across to Astoria. Climb the Column for beautiful pictures of the Columbia river and surrounding area. Head south on US101 to Florence and then East on Oregon 126 to Eugene...we did this trip two years ago. September will have the kids back in school and you shouldn't have trouble finding spots along the coast. If you decide this route, I can give you campgrounds suitable for your rig.
For a more direct route, take I-90 south from Spokane and US 395 south out of Ritzville. Once you get to the tri-cities area, there's a KOA there but nothing fancy, just expensive, stay on US 395 to Plymouth. Stay at the COE park alongside the Columbia River. FHU sites right on the river @ 20/night/10 with a geezer card. From there you have two choices. Washington 14 along the north side of the river or I-84 on the Oregon side. We chose Washington 14. Easy drive, less traffic and more to see. We stayed at Memaloose State Park. Plenty of room for a big rig. Be sure to stop at the Bonneville Lock and Dam and watch them move the barges and boats up the river. We crossed the River at the Hood River bridge because there were warning signs along Wa. 14 of low clearance tunnels. As you head west, be sure to cross the Bridge of the gods, if for no other reason but to say you did it. Just before Portland, pick up Interstate 305 south. It bypasses Portland altogether and will bring you over to Interstate 5 and south....
Monaco has on-site facilities, water/elec with a dump station. If you have a macerator, try to get sites 1 or 2. A 50' hose means you don't have to move your rig. :B
Good luck...Dennis