You will have a problem with a 38' motorhome. In our references for the Olympic NP campgrounds, none are rated for more than 25'. Kalaloch is indeed nice, but I don't remember any camp sites that would handle a 38-footer. We squeezed in with the 31' we were in at that time. The only campground you might fit in would be Fairholm campground west of Port Angeles. I just can't remember if it is run by the NPS or the Forest Service. We drove through it and saw several sites that would take larger rigs. Kind of tight in places.
Some other options... There are Elks lodges in Sequim and Port Townsend, if you're a member. There are WA state parks at Port Townsend and Sequim which are absolutely cool parks, if somewhat pricey (Fort Worden SP, Fort Flager SP, and Old Fort Townsend SP). There is also Dungeness State Park, which is a county-run rec area/campground on the coast north of Sequim which is pretty nice. There is a lower-cost campground at the country fairground in Port Townsend, as well as a more pricey city campground right on the waterfront.
Since we didn't use any commercial cg's when we went through there in 2007, I can't give you much help. They are there, but you'll need to do your own research!
Still, its a beautiful area and we would go back anytime! Have fun!
Bryan & Susan
2007 Pacifica TE-40B