Newhalem Creek's campsites sit under old growth down in the Skagit River gorge, and the river runs right beside camp. That's where the rig spends the first night. The next morning SR-20 climbs to the overlook above Diablo Lake, where the water below is a milky turquoise that looks lit from underneath. From there the highway keeps going up to Washington Pass, the loop's high point at 5,477 feet, and then drops east toward Winthrop.
The loop suits a family motorhome because the nights sort themselves by side. West of the pass the campgrounds sit inside North Cascades National Park and run dry, and east of it Pearrygin Lake State Park has the hookups for the back half of the week. Home is a different road, south on US-97 and back over Stevens Pass on US-2, so the last day's driving repeats none of the first.
Newhalem Creek and Diablo Lake
Newhalem Creek is the campground to build the week around, a forest camp at the bottom of the gorge where the kids can wander down to the Skagit after dinner. It's also the park campground that takes the longest rigs. Recreation.gov lists its sites from about 30 feet up to 70 feet, non-electric with no hookups, so the site you land decides how long a motorhome you rent. Reserve the pad first and the rig second.
The second day belongs to Diablo Lake, and Colonial Creek has the campground right on the water. The Park Service listing for Colonial Creek North says most of its campsites don't accommodate large trailers or RVs, so a family motorhome comes for the shoreline and a picnic and sleeps back at Newhalem.
Those park nights run dry from start to finish. The Park Service lists no services inside the park and names Marblemount as the nearest, so arrive with the fresh tank full and the holding tanks empty. Colonial Creek North lists a dump station for the drive east, and after that the next hookup is Pearrygin Lake.
Over Washington Pass to Winthrop and Pearrygin Lake
The switchbacks under Washington Pass are slow, low-gear miles in a motorhome and the best seat in the rig for whoever isn't driving. On the far side the forest opens up, the air dries out, and Winthrop's main street of wooden boardwalks is where the family climbs down for lunch and a stretch.
Pearrygin Lake State Park, just outside town, is the base for the back half of the week, a campground on a shrub-steppe shore where the dripping forest of the Skagit side already feels a long way off. Its hookup sites and trailer dump reset the rig after the dry park nights, and everyone can shower without counting gallons. Washington State Parks takes the reservations and closes the campground to camping from November through March.
When SR-20 is open
The whole loop depends on the pass, and snow and avalanche danger usually close SR-20 from late November or early December until April or early May. That makes this a week for late spring through fall, with the park campgrounds booked by reservation in summer and first-come after it.
Closer to Everett, on roads that never cross the pass, the tide races through the channel under the Deception Pass bridge on the way onto Whidbey Island, and the Mountain Loop Highway and the trail to Wallace Falls fill a weekend in the foothills. Both suit two people in a smaller rig, and Everett's smaller-rig page covers them. The rest of the state's motorhome weeks, from the Olympic Peninsula to Mount Rainier, are on the Washington RV page.