It's legal to walk on interstates in OR, but not in CA. If you do it anyway, chances are you'll get a ride before highway patrol comes by. It's enforced less on the I-5 then on the expressway sections of 101. Seen people get kicked off near Eureka/Arcata.
Ain't got specific places to recommend camping, but as one person with just a pack you'll have an easy time. In the north you got thick forest to hide in, further south tamarisks sprout up anywhere untended and give good cover.