Another campground worth considering is the
Silver Lake County Park near Maple Falls, WA. About 15 miles from Sumas. Four campgrounds, some sites with hookups (water & electric) & some dry camping. Not much in the way of stores or restaurants nearby, but a very nice park.
This is a great location for a visit to WA 542 (the Mt Baker Highway). If you haven't seen the view from Artist's Point at the end of the highway, you should, however it would be a good idea to leave the trailer at the campground (a bit of a climb at the end of the road).
More photos. One note - My visit in late July, 2012 still had snow on the ground & heavy fog (see the link in the photos). The page of photos linked above was taken at the same time of the month in 2014 and it was beautiful.
There are also some very nice Forest Service campgrounds on the Mt Baker Highway, but I've never been able to get a reservation. Dry camping.