Right now they're having some snow in western SD, but in a few days things should be better. You will perhaps have cool rainy weather, or a nice spring day, be prepared for whatever. Check the rapid City Journal for weather reports and current conditions (rapidcityjournal.com). It's a little early, but you will probably be fine, I don't know of any CG's that are open where you can stay, sugggest that you visit Mt. Rushmore and the Needles Highway in the car and leave the RV somewhere convenient. Custer SP is also worth a visit, don't know if their CG is open or not.