We've made many Montana to Puget Sound trips and Ritzville to Moses Lake are good distances for a stop.
There is a small RV park attached to a Hotel in Ritzville (the name keeps changing, last time it was a Travel-Lodge) that we have stopped at several times. It's not fancy, but it's shaded, has a pool and you have access to the Hotel. There are two places to eat right next door. They have wifi, but it's better inside the Hotel.
In Moses Lake we have stayed at Cascade Campground (city, no wifi or TV), Grant County Fairgrounds (dito) and Desert Oasis RV. Again places we would stay for an over-tnight.
There is (was?) also a Coast-to-Coast membership campground in Moses Lake, we have stayed at once.