Hi Dr. Mom,
I would suggest you plan 4-6 days in the Black Hills (BH), 3-4 days of seeing the sights and 1-2 days to just relax. Would you consider renting a car for a few days while there? I would not want to drive a big long RV on some of those windy roads, although you could probably stick to main roads and see plenty too. If it were me, I'd park the RV at a campground and rent a smaller vehicle for 3+ days. Yes, it's an added expense, but maybe less so (and a lot less hassle) than towing your van along. It might be a gas savings too?
I live on the eastern side of SD. Grew up here, moved away and then we moved back to the area to raise our children. As a child, I spent many summers in Spearfish Canyon (northern hills between Lead & Deadwood) at my grandparent's cabin. The cabin is no longer in the family, but we've taken our kids to the Hills 3X in 10 years. Some years, they've chosen BH over WDW or DL. BH is a very relaxing vacation, in our opinions. It helps that it's only a 6 hour drive for us (though it takes much longer, with all the stops we like to make.)
The Corn Palace is in Mitchell. Nice stop along I-90. A good place to stretch your legs, have lunch or an ice cream. I think there is a KOA Campground near Mitchell too. There is a
Walmart just south of the interstate and a Cabela's, among other stores, etc. The CP is in interior of the town (follow the signs) toward the north. True that the attraction of the CP is mostly on the outside and doesn't take long to see, but it is charming and I think your kids would enjoy seeing it. If they are working on the artistry, it is fun to see the different colors of corn they use in the artwork. The CP is on the town's main street, so there are several shops to walk to as well. There is a gift shop inside the CP, along with an auditorium and a gym. We've been to several basketball games, concerts and school musical competitions there.
Further west near the Missouri River are Chamberlain (east side), and Oacoma (west side). Be sure to stop at Al's Oasis for a buffalo burger and a slice of pie. It's been there forever (still owned by the same family, I believe). I don't think we've ever gone out to the Hills without stopping. There is also a gift shop and a grocery store, as well as a couple of gas stations near by. If you'd rather picnic, there is a scenic rest stop on the east side of the river, along the interstate.
Wall Drug - also a must-stop, IMO. Yes, it is kitschy, but that's part of the charm. As another poster mention, FABULOUS donuts, especially when they are still warm! Pretty great ice cream cones too. Creaky wood floors, trinkets of all kinds and vintage life-sized cowboy dioramas. When I was a kid, they were coin operated, but I think now they just play sporadically on their own. Most of them sing too! Fun to watch
I would allow at least a couple of hours at Wall Drug. Check out their website and see the map. The store is divided up into different zones. Some are outdoors and in different buildings. Along with just about every souvenir you could think of, there is a western wear store, fudge shop, book store, rock shop, and you can even pan for gold (building out back). Lots of great photo ops, including those cardboard cut-out things where you stick your head through the hole. Don't miss the Giant Jackalope! And, yes it is Wall DRUG store, complete with a working pharmacy. Their "FREE ICE WATER" line was the draw for people back in the 1930s to come off the main road and stop there. There is a drinking fountain/well out back. So, they do still have FREE ICE WATER, but we thought it tasted pretty awful. It was COLD though!
Badlands - yes, they are beautiful. But, it is worth the windy roads to drive all the way through it? Not so sure. Since you'll be driving an RV, I would suggest taking part of the loop and stopping off at the first overlook. Get out, let the kids run a bit and take some photos and go back the way you came. Yes, you'll miss some of the beauty, but you'll also save a whole lot of time and you'll still see some of it. JMHO.
I don't think you need to buy a guide book. There are plenty of websites and travel booklets you can get online/order. Go TravelSD.com, order a guide book and see what they have on the website. Also VisitSpearfish.com.
There is a Good Earth Natural Food Store in Spearfish that you might enjoy.
Places I've been that you may want to consider:
* Near Rapid City - Rushmore Waterslide, Black Hills Maze, Reptile Gardens, Bear Country
* Keystone - Presidential Alpine Slide, Taffy Shop (Watch them make it fresh!). Note on the Slide: you take a chairlift to the top and then have the option of sliding down or taking the chairlift back down. If you do this, be sure to get off the lift and walk around a bit before heading back down. Your kids would enjoy picking up mica. There is also a snack shack (burgers, etc) and a be sure to take a camera for the awesome views of Mount Rushmore - and wood cut-out photo op of Mt. Rushmore with a slot for a 5th face (your child). This attraction might not be appropriate for your youngest children, but your older children might love it (with one parent?)
* Deadwood - Adams Museum - my kids still remember the 2-headed cow!
* Spearfish Canyon - Bridal Veil Falls, gorgeous drives (but I might be biased
) I think there are a couple of fishing places too.
* Cosmos Mystery Area - not far from RC and Mt. Rushmore
* Crazy Horse - especially at night
* Mount Rushmore - be sure to take the walk up to the faces. You miss so much if you don't.
I don't know much about campgrounds as we've always stayed with relatives in their cabins or rented a cabin in the BH. FYI, Rim Rock Lodge in Spearfish Canyon has some charming cabins. Not much has changed in 40 years, so it's not fancy, but we like it.
I see that this has gotten extremely long. Sorry about that, but I hope you find some of it useful. PM me if you have any specific questions and I will try to help.
I've read some of your TRs and hope that you choose to do a TR on this trip as well! Have fun!
Custer National Battlefield is worth a trip, and it is REALLY in the middle of nowhere. But, you get a real sense for what happened there, and Custer comes off even more foolish than the history books made him seem 
The highway into the NE section of Yellowstone (Beartooth Pass) is breathtaking!
).
Keystone is a bunch of gift shops but it is fun to walk around. 
