This RV resort is situated on the banks of Silver Lake, and is about 9 miles East of I-5 junction 49. It is a very short drive from the Mount St Helens Creation Center – indeed, you can walk the distance in about 30 minutes!
Please see the resort's website below for price details for RV pitches. The resort also has a number of wooden cabins available for rent, which are pleasant. See also the link to Trip Advisor, though at the time of writing (January 22nd 2015) there was only one (favorable!) review on the listing.