I maybe late to the party, but I would vote with those recommending 22 from McMinnville to the coast. Although I have driven many Oregon back roads with and without the coach, I am not a fan of 99w to 22. Probably just me. The people at Olde Stone have a map that they recommend to get back to (or from I-5). I don't remember the exit number but it is the Donald exit. Exiting I-5 there and going south is, I think, a better choice. When going to McMinn from the north I always use that route. There are also two good truck stops if you need fuel.
On the other hand, I would not avoid 20. I drove it with the coach in July. The challenge is the last 10-15 miles where there is a good deal of incomplete construction with narrow lanes and sharp curves
In any case, enjoy the trip.