You should have no trouble with the North Cascades (Hwy 20). Washington Pass and Rainy Pass are two of the highest passes in the state. Take you time. I would travel during the week as opposed to the weekends. Less traffic. You will find virtually no semi's. Might see a logging truck or two. If you have time, stop at River's Edge RV park in Twisp. If FHU's are not an issue, Early Winters and Ross Lake are nice. There is also a great state park just west of Diablo. There is also another nice park in Concrete. A former KOA but still nice. Do make sure you have enough fuel. Services are few and far between once you leave Winthrop. Oh and there is also a great state park outside of Winthrop. The picture on my sig. was taken at Ross Lake. Do the City Light tour if you have time. They use to do a fried chicken to die for. It is one if not number 1 most beautiful drives I have EVER taken and that includes Going to the Sun and Yellowstone.
I try and do it at least once a year just to get myself 'centered'.
Tom, Cheryl & Blossom(coonhound mix)
'05 Winnebago Journey 36G w/Cat. C-7 350 hp Freightliner XC
AFE air filter, aero turbine muffler, 4 FSD Konis, ultra track bell crank and Safe-T-Plus
I can think of 3 NF camp grounds directly on 20, Loup Loup east of Twisp, Early Winters and Lone Fir (?) on the climb toward Washington Pass. Others involve some degree of gravel road travel.
At Lone Fir the sites closest to the highway should be large enough and accessible, but it would be a good idea to walk the loop before committing a large RV to it. Parts of the loop are tight, if memory serves.
The state park near Winthrop is very popular with the summer weekend ground (its on a rec. lake); so check reservations.
National Park campgrounds on the west side are nice, and relatively inexpensive.
Look up the Nat Park, and the FS districts (Mt Baker on the west, Okanogan on the east). Also Washington State Parks.
That loop below the Liberty Bell (east side of Washington Pass) is not going to catch you unawares. At the top there is well signed turn off to an overlook, which includes a view of this descent. Then there's a long descent under the Liberty Bell, with this loop at the bottom. You see everything.
Coming the other direction you also see everything.