Walt Flanagan , Jun 20, 2010; 02:02 p.m.
Look at the map of Yellowstone. There are basically two loop roads that make a figure 8 and roads to the N, S, E, W, and NE that enter/exit the park. A lot of the road is very curvy with lots of elevation changes. I wouldn't want to be driving constantly in an RV but some don't care.
Check the park websites for road closures. A year ago part of the loop road was closed which turned a 15 min trip into 3.5 hours.
Here's a picture of Craters of the Moon NM. There are multiple lava fields around that are pretty interesting. You can spend half a day there and go on.
(Craters of the Moon National Monument -- 1000 x 667 photo)