look up "benchmark maps" they publish the best maps for the western u.s. states. these are very useful and indicate all paved vs gravel surfaces with high reliability. they also show a lot of detail on campgrounds, etc..
THe route from Mt Rushmore , west has a lot of opportunities for you across Wyoming and toward Cody.. then up the Chief joe hwy and over the Beartooth hwy, then up toward Great Falls and Choteu, east Glacier.... IMO.
Lots of threads on trips like this... Have fun!