Snow usually comes to the High Desert in November, but is a possibility in October. Metro Portland has few RV parks, except for one sizeable one in Troutdale to the east. Some RV parks line I-5 in Southwest Washington. There are good options in the Willamette Valley north of Salem, especially State Parks, but in all cases check in advance for seasonal closures.
Staying at Troutdale gives you the option of heading over Mt. Hood's south corridor, Hwy 26, to Bend - and it gets you off I-5 sooner; hit I-205 southbound north of Vancouver to work your way to Troutdale. Or you might get on Washington Hwy 14 for a drive up the Columbia River's north shore to Maryhill, where there is a great state park on the river, then the next day cross the bridge there toward Bend.
You could also go east to Hood River on I-84 and take Hwy 35 over the east side of Mt. Hood to 26 and to Bend, or hit Hwy 97 at the Dalles. I used to go the Salem route to Bend regularly, mostly because I enjoy the Metolius River at Camp Sherman, east of Sisters, enroute. But since I live close to it, I hit Highway 26 over Mt. Hood these days, to avoid the freeway altogether. All roads mentioned are good and the scenery enjoyable, especially in the fall, and will appease your quest to get off I-5.
I'll let others speak to the south half of your route. We may cross your path in Bend as we head home from our summer-long voyage.
Joel