Not to be contentious, but there is no good fueling in either Newport, Phillomath, or Corvallis for a 42' motor home pulling a toad. The best bet (that I'm aware of) is to go over to I-5 via 20 and 34. At the intersection of 34 and I-5, there is on the east side a 76 that is very doable and also a CFM (I think) truck station beside the 76 that will take public vehicles. Both of these are easy in and easy out.
Or, once on I-5 going north towards Portland, just north of Salem is a Pilot/FJ station.
Also, if you want to go 99W out of Corvallis, you first have to go through downtown Corvallis. Not a good idea.
The shortest, easiest, most direct, and have access to easy diesel....is 20 to 34 to I-5.
Ron