If you have any interest in seeing Howard Hugh's "Spruce Goose" plane that he built and flew just before WWII ended, it's now in an excellent flight museum in McMinnville, OR. That's about 60 miles SW of Portland. From there you can travel the highway out to the Oregon Coast near Seaside and then up to Astoria.
As late as your leaving make sure you take winter clothes, and have tire chains for your rig. Some mountains will have snow by then. Also I grew up in Astoria and am now a truck driver. If you come into Oregon from the east stay on us 26 or I84 to I5 take I5 north to Longview WA then fallow truck route across the Columbia river to US 30 that will take you into Astoria. Someone mentioned WA 4 (dont use it if your over 40 feet total length) it's got some curves and low clearance. Hwy 30 on Oregon side of river is I84 east of hoodriver west of hoodriver you dont want to take an rv on it and part are still closed from the wildfires last summer. If you come into Oregon from the south hwy 101 is a nice drive for views. But traffic sucks and the road is in poor shape at best. I would take I5 to eugene or salem then cut over to 101 and head north. Road still sucks but you bypass Portland and it's worth it. Like other have said the mountain passes are nasty high. Mountain out your way are 5k to 6k feet. There a hill or a Butte out West. Our mountain passes out here range from 5000 feet to well over 12000 feet in Colorado. Just beware if your pulling a big trailer or have a gas motorhome slow down take your time on passes stay to the right and run with 4 ways on. On the down side make sure you down shift your tranny and stab brake 3 to 4 seconds on 3 to 4 seconds off the brakes or you will over hea4 your brakes and risk catching them on fire. If you see light smoke coming from your brakes when you get to the bottom DONT STOP for a few miles they will cool off on there own. You see fire yes stop and use fire extinguisher. On them.