We traveled part of that route a few years ago. We can share some of our experiences and suggestions with you.
We stayed at a free campground at Haugen, MT. It's the Silver Dollar Casino at Exit #16. They have 30 amp hookups, but no water or sewer. It was OK....for free. The restaurant at the casino was not great.
We liked Mountain View RV park in Sundance. We got their Passport America rate of $16.50 The water there was great. Normally we would buy jugs of drinking water along the way, but in Sundance, we filled our water jugs from the tap, it was that good. We took a day and visited Devil's Tower, worth the drive.
If you're into aviation museums, there's The South Dakota Aviation Museum at Ellsworth AFB near Rapid City, not too far off the freeway, and their parking lot is plenty big for RV's
In Mitchell,SD, we stayed at the R&R Rv park. It was an OK campground for $15. We visited the Corn Palace, kind of an interesting place.
If your route takes you near the Twin Cities, I'd recommend staying at Dakotah Meadows RV Park in Prior Lake, just southwest of Minneapolis. It's a very nice park for just $26/night. Just make sure you ask for one of the single sites up the hill along the back row. (a lot of the other sites are back-to-back with shared hookups.)
It's an easy walk to the Mystic Lakes Casino, a very nice "Las Vegas" style casino with an excellent atrium restaurant.
That was in 2011, our route from there took us south through Iowa.
Hope you have a great trip.
