He confessed to 2 murders in Oregon, and only one victim was identified. The known victim is Roberta Parks. He abducted her from the Oregon State University in May 1974. If you check the distance between Seattle and the Oregon State University it’s “only” a 5 hours drive, and as you said, with all the gas he put in his VW he didn’t need to to stop to a gas station while there at the time.
In September 1974 he made another stop in the State and made a gas purchase in Pendleton, and again on June 6 and 12, 1975. My best guess is that when he took his car to travel from SLC to Seattle or vice versa he would pass through Pendleton and he might have had a victim while there (a hitchhiker maybe?)