This was by far my favorite place we stayed on our entire 4000+ mile road trip…. and it was free! We stayed at Priest Hole, which is a few miles past the Painted Hills visitor center on dirt roads. We saw a few vans and a pop-up trailer, but no RVs. The road was okay but not great, so not sure if regular RVs would make it or not. Fine for cars, trucks or vans.
It's Bureau of Land Management land, which means all camping is free. We camped right on the banks of the John Day River and it was amazingly beautiful. The water was the perfect temperature for swimming and crystal clear. There were people a couple hundred yards from us on either side, so not too crowded. We were there on a Thursday and pulled right up to an empty spot.
Only vault toilets. Bring your own water. No showers, picnic tables, etc. The only drawback, besides lack of a nice bathroom and not much natural shade, was quite a few earwigs and other bugs attacking the outside tent at night when we had our light on. Not the worst thing, but slightly annoying for the kiddos.
When the river is higher (sometimes in the winter) there's a large grassy area in the center of the area for tents, but a bit far away from the water. But in the summer when the river is lower you can camp in any of the rocky dry stream beds and be safely right on the water.