I would like to second what some posters said about Grizzly RV park in West Yellowstone. That's where we stayed and took day trips into Yellowstone. It's a neat little town--the Wolf and Grizzly Discovery Center is well worth a visit--there is also a wonderful welcome center just before you enter the park and the rangers will provide you with all sorts of maps and information.
Devils Tower is something to see--I walked the 1.5 miles around it...and the road leading up to the tower has an area called 'Prairie Dog Town' where you can stop and see hundreds of the little dogs running around and popping in and out of their little holes.
We stayed at the Rapid City/Mt Rushmore KOA and there is a lot to see in that area besides Mt Rushmore.
Don't know if it would be on your way, but Multnomah Falls in Oregon is spectacular.
And I don't think there is a bad place to stay along the Oregon coast. Hwy 101 is one of the most beautiful drives ever.
Your trip will be wonderful!
