I saw at another thread the following so I think Clarkston to Ellensburg is doable.
We have driven from Clarkston to Ellensburg at least 15 times in all kinds of weather. It is net uphill, and fairly often against the wind. In very cold weather (subfreezing) the 200 mile trip can be a stretch. More than once we have arrived with single digits rated range. If it is cold and/or a headwind, you CANNOT drive the speed limit or above and hope to make it. We take the shortest route (and prettier) through Connell.
A couple of times we have made an insurance stop at the lovely little Tucannon River RV Park, just east of Starbuck. The drive north from Starbuck is a fabulous twisty-fest, if you are in the mood!