When I go to Spokane I break up the drive by going to Wild Horse Casino in Pendleton, OR for a day or 2. They have an RV park (nothing to write home about), commercial truck parking area, an RV parking area, and an Arrow gas station. I use the RV parking area with the other RVers. The casino has a nice buffet and a 5 theater cinema. The buffet has a nice view of Cabbage Patch Hill. I don't know what their present restrictions are.
I'm in Chinook right now. This is the route I would take:
101 South to Astoria, OR
OR-30 East to Longview, WA (433) - (it's easier than driving WA-4 on the WA side of the Columbia)
In Longview - 433 to 432 to I-5 South
I-5 South to I-205 South to I-84 East to Pendleton, OR
Then double back 30 miles on I-84 West to I-82 West
I-82-West to I-395 North to I-90 East to Spokane
I have found that Biggs Junction, OR is an easy in/easy out location for fueling.
I-395 is a really nice road. You can set the cruise control and enjoy the drive.