You didn't state where you're coming from but a good route would be to somehow in your drive connect to Hwy 97 in California and head to Klamath Falls, Oregon. An excellent highway in California is Hwy 395 on the eastern Sierras. It will avoid all the California traffic around Los Angeles and San Francisco. It's a gorgeous drive with nice places to stay, both private and public parks.
You could take it north to Alturas and pick up 139 to 39 into Klamath Falls, Oregon. A little further north is a wonderful state park very suitable for big rigs called Collier Memorial. It's a good base for touring Crater Lake.
Then a little further north on 97 pick up 138 taking you into Roseburg, then north on I-5 to Sutherlin and picking up 138 to Winchester Bay on continue up the coast. These are all good for RVers
When you leave the Oregon coast if you're planning to head south again you could reverse the above route by crossing the Astoria bridge into Washington (for the experience of driving the bridge) and picking up Hwy 4 east to I-5 at Longview, WA and head to Portland, OR. Pick up Hwy 26 taking you to Hwy 97 toward Madras/Redmond/Bend/Klamath Falls, Oregon for Crater Lake when you're finished with the coast and continue to Hwy 395 in California to wherever you're heading next.
To confuse you more, you mentioned Crescent City, CA..how are you getting there? If you're on Hwy 395 in California pick up Hwy 50 around Lake Tahoe and take Hwy 50 or even I-80 into Sacramento. If you're planning on touring San Francisco, stay in Napa at the RV Expo park. Then take Hwy 37 at Vallejo to Hwy 101 at Novato and head north on 101 to Crescent City and the Oregon coast. You can pick up Crater Lake on your return trip as I stated above. Hwy 101 is a RV-friendly highway up the coast (not to be confused with Hwy 1 which is very narrow in California). We enjoy staying in Redcrest, CA at Ancient Redwoods RV for touring the big trees. You're right in the midst of them. 101 all the way up through Oregon is a beauty!
Even if the whole rim road of Crater Lake isn't open yet because of snow, they always try to keep the road open to the lake itself and the visitor center so at least you'll be able to see it. The last time we were there we could drive part of the rim road before the snow block. Each year is different as to when it opens fully but just seeing part of it is worth it!
Personally, and again, depending on your whole trip plans, driving up CA395 to OR97 to Portland and then over to the Oregon Coast is a better direction of travel for doing the whole coast and taking you into Calif on Hwy 101. You'll be heading south for better expansive views and the scenic pull-offs will be on your right so you can pull off and not have to cross traffic. Have a great trip!!
