Others have provided good input for alternatives. Just to add...
US 16 Cody to Buffalo is a good drive with diverse scenery. You won't encounter major climbing until you get to Ten Sleep (a funky little town worth a stop to see the town). From Ten Sleep heading east you'll climb to Powder River Pass, which tops out just shy of 9700 feet -- not overly high by Rocky Mountain standards. It's not a difficult climb, but be aware that it's a favourite of motorcycle riders so there may be a goodly amount of MC traffic. Once you get to the top, it's a beautiful, meandering drive down to Buffalo, and it's not steep at all on the east side of the pass. You'll go past a lot of ranching land, several dude ranches in that area between the pass and Buffalo, and IIRC, some of them have sites for RVs if you might be interested in stopping there. OR you could stop in Buffalo, several good RV parks there. You can also grab I-90 in Buffalo and take it over to Devil's Tower.
