To get a really good experience at Devil's Tower, you need to do the shorter hike around the base as you get so many great and different views of the tower. Little Big Horn is interesting to see, and you could can do quite a bit in a couple of hours.
While I believe there is RV parking at both locations, the problem could be with the amount of people and spots available. From what I saw either location could be a problem. My suggestion would be to stop and see Little Big Horn and then move on down to Devil's Tower. The best time to see and/or hike around Devils Tower is either early in the morning or late in the day. The KOA right at the entrance of Devil's Tower is very convenient and allows for multiple trips into the park.
If I had to choose just one of the two, I'd go with Devil's Tower but I'm biased due to my interest in photography.
