MT Saint Helens visitor Center

I think I have the "Mt. St. Helens Vistor Center" (state of Washington) confused with the Johnston Ridge Observatory. The one I am talking about is five miles east of I 5 on Wa 504. We are considering a stop with the camper on the way south on I 5 if it is worthwhile to stop here or to make a day trip without camper from Troutdale, Oregon all the way to the Johnston Ridge Observatory. Thanks,