Swift Creek ($18), Panorama Point ($16), and Park Creek ($12) all have some sites that will easily handle units with a total length of 35' or so. We were much less impressed with Shannon (north end of the lake - about 8 miles of gravel), Horseshoe, and Boulder Creek.
There are five boondock sites at the lake shore at the end of Forest Road 1122. Three are barely large enough for a Scamp or Casita (and the others are smaller) but they also seem to be quite popular. It was my impression that they require a Northwest Forests or a Senior Pass. There are a few other boondock sites in the area but I would suggest dropping the trailer before exploring some of the back country options.
Skagit County has a primitive campground at Grandy Lake (not far north of Highway 20 on the Baker Lake Road) that was clear full when we went by. Apparently it was a "free fishing" weekend.
Howard Miller Steelhead Park is located in Rockport and owned and operated by Skagit County. It has elec/water hookups at approximately 50 sites (@$30) and a dump station. A few sites are directly on the bank of the Skagit River. Very clean, lots of well trimmed grass, showers, and no privacy between sites.
The County also has another primitive park in the area. The Sauk River Park Campground appears to be a boondock site for up to 15 units, located south of Highway 20 on Highway 530. There is apparently a summer-time fee but we haven't checked this one out yet.
Newhalem Creek ($12) is a relatively new campground with three large loops and a dump site near the entrance. All sites have paved driveways and most have very nice, large tent pads. Some sites are large enough for 40' or more total unit length. This is site 31 in the B loop.
Both of these campgrounds are located just west of Newhalem, have very good Verizon 4G coverage, and offer 50% Senior Pass discounts. One loop of Newhalem is reservable sites - the rest are first come, first served.
Gorge Lake Campground is located just west of the company town of Diablo near where Highway 20 crosses Diablo Lake. There are six sites and no water. A couple of the sites would work for units of our size. The best part on this one is the price - it's free.
Colonial Creek Campground ($12) is a large campground (144 sites) split by Highway 20 where it crosses the Thunder Creek Arm of Diablo Lake. It has a dump station near the entrance on the south side of the highway. Some sites are up to 35' long and almost all are very shady. A handful have direct access to the water of Thunder Creek Arm. There is no Verizon coverage here but the Senior Pass discount does apply.
There are three Okanogan-Wenatchee National Forest campgrounds on the east side of the pass. Lone Fir Campground ($12) is nearest the pass, then Klipchuck Campground ($12) about 18 miles from Winthrop, and Early Winters Campground ($8) (on both sides of the highway) about 16 miles from Winthrop. Early Winters would have the most road noise.
In mid June and mid week, we've found all of these campgrounds nearly empty.