Any suggestions for motorhome day trip to view August 21, 2017 eclipse from Fossil, Oregon?

I'll be driving our family of 5 south from Yakima, WA in a motorhome to view the eclipse. We have a family event Sunday evening in Yakima and plan to get started for Fossil early Monday morning. Normally the drive would be less than 3 hours.

- Should we expect glacial traffic due to clogged roads? Is it unrealistic to expect to get there in 3 hours? Would it make more sense to drive late Sunday night?

- We don't expect to be able to find a campsite. Where can we find good places to pull off of the road and view the eclipse from there? Or spend the latter part of the night, if we drive Sunday night?

- Should we expect glacial traffic driving back after the eclipse?

- If we can't get back to Yakima by evening, are there good places along the way to park the motorhome overnight? We don't need a campsite nor hookups.