The drive from WA14 at Carson up the Wind River Hwy and then on Curly Crk Rd to FS90 has one of the best east side viewpoints of Mt St Helens. It's a double loop as Curly Ck drops to the Lewis River.
The Lewis falls and campground are to the east on FS90 (earlier I wrote FS99). These are all paved two lane FS roads. Not as well signed and graded as state highways, but still quite driveable.
It's FS25 that runs north from FS90 to US12. That has some twisty sections, but no curve is too tight for RVs (after all they were built with logging trucks in mind). FS99 is the side road to Mt St Helens. Iron Ck is a nice FS campground on 25 just south of Randle.
I have also enjoyed exploring the Nat Forest closer to Mt Adams, with the most challenging (but enjoyable) campgrounds being Bird Lake on the Yakima Reservation on the southside of Adams.