Stay as close to the mountain as you can get from the west side at 
Eco Park at Mount St. Helens (just past mile marker 24, 360/274-6542). Owned by the same family who once ran the Spirit Lake Lodge in pre-eruption days, Eco Park offers the only accommodations inside the blast zone. The resort has summertime basic cabins ($100–125 d), yurts ($75 d), and campsites ($17.50) in a remote area on the northeast side. RVs are accepted on the site, but there are no hookups.
Toutle Diner and RV Park (5037 Spirit Lake Memorial Hwy, Toutle, 360/274-6208) offers full hookups ($21), some sites with cable TV. The diner (Sun.–Thurs. 8 a.m.–9 p.m., Fri.–Sat. 8 a.m.–10 p.m.) offers typical roadhouse fare, along with espresso and free Wi-Fi. The property also has shower and laundry facilities.