Lots of good advice given. My advice is to hit Yellowstone as early in the morning as you can. It's the difference between having the road to yourself and enjoying the park, or sucking RV exhaust.

 

Make sure you top off your tank whenever you see a gas station, they can be few and far between out there. Best riding I've ever done bar none!

 

Some good advice given in this thread. I live north of Rapid City about 2.5 hours and try to get down that way pretty often. You can't go wrong in this area of the country, you should try checking out the Badlands and Scenic, South Dakota area. I would say it would be worth it to check out devils tower while you were in the area, just to say you have been there. Pretty cool place. Enjoy!!

 

You have a lot of advice here and can not go wrong anywhere out there. Its beautiful