Since you're taking two days, take the scenic route and drive south to Lewiston and then cut through Walla Walla and down to Pendleton and then take 395 through Ukiah and Dale and then take 26 west to Prineville.
There's a ton of forest service camping to be had in that stretch from Ukiah down through Dale that's beautiful and free, and far nicer than any RV Park. You're only overnighting it so hookups shouldn't be an issue.
And actually while I'm thinking of it you could cut off of 395 at Long Creek and then cut over to the west to Kimberly and then south to 395. That would take you through the painted hills and you might even score a famous peach around Kimberly. I'm not sure when they harvest those, though.
