I'm not as familiar with things on hwy 20, but that will take you through Bend and I believe there is a nice Oregon State Park near there. Plus, there have got to be plenty of nice RV parks too. It's a big, outdoor recreational area.
As far as 26 is concerned, there is a great place called Kah Nee Ta Hot Springs and resort that has a fabulous mineral springs fed pool at their RV park. It's a 20 mile detour off of 26, but might be worth checking into. Plus, once you get to the Mount Hood National Forest, there are going to be about 4 heavily forest campgrounds that you can camp in. No A/C needed at that elevation. If you want something with full hookups though, in Government Camp is the Resort at the Mountain which is a nice RV park with a pool and stuff. Once you get off 26 and head toward WA on I-84 in Oregon, if you needed to stay near the town of Gresham, I would recommend Portland Fairview RV Park located at exit 14 off I-84. There is a Camping World, Love's travel plaza, Walmart Supercenter all within a few mile radius. Hope this helps!
