If it were me, I'd skip the 5 altogether. While it is an interesting experience, it's not that popular for RV-ers and there's not a whole lot of CGs along it plus you're battling the truckers all day long. The OR coast would def. be recommended.
I'd suggest taking an inland route up into the east side of the Cascades. Take 97 eastward off I-5 from Weed up to Bend and from there take 26 to the east side of Portland. These roads are an easy drive. From Portland you'd have a choice of several routes to the coast depending on how far north you'd want to start from. The scenery and environment on the eastern side of the mountains is really different than along the coast and you'll see the diversity Oregon has. Crater Lake is just off 97 too.
If you do end up on I-5, I really like the drive from the north side of Grants Pass up to Medford/Ashland and down to the south side of the OR/CA border. Have camped a few times in the Siskiyou NF just east of Ashland and it's nice up there and there's a number of CGs there.