We have never camped in Seattle, other than the Olympic Hotel, a comfortable place, but tend to go on north to Bellingham and stay at the
www.bellinghamrvpark.com. Nice stop, easy off and back on to Interstate 5, but far enough away so that road noise isn't a problem.
Some of our family live in B'ham so we on occasion will spend a week or so here.
Then we go north and cross at Sumas. There is a real nice KOA in Lynden, we then still use the Sumas crossing. We tend to stay in this area more, on our way southbound, after spending the summer in the north country.