I'd recommend SR20 rather than the others. We haven't gone over the mountains on SR20 since we sold the RV in 2010. Towards the end of that season, the major improvements to the highway were well along. It's a very scenic route and you get to see the surrounding scenery much more easily, as in most place the sight-lines are excellent.
Coming up the highway from Winthrop to the top of the pass is a bit of a drag. Our 32-footer (no toad, two people and two Labradors) held a steady 40 mph. It was an '02 with the V-10.
The pass, which is at 5,200 feet, typically closes in mid-October when the snow gets to be too much to keep plowing. The look-out area at the top give wonderful views, as it;s at about 7,000 feet.
It has lighter traffic than either US2 or I-90 and you don't get the feeling of driving in a trench like on US2. You have a longer stretch of I-5 to cover south to Seattle from Burlington than the other passes, but it's a good road with not much congestion except around Everett and getting into Seattle.
Frank Damp -Anacortes, WA,(DW- Eileen)
ex-pat Brits (1968) and ex-RVers.