Here's my $. 02:
1. Plan a stop in Rapid City, SD (off I-90). Mt Rushmore is about 20 miles south and is worth the stop. You can tow your RV up there (roads are good) but plan on arriving in non-peak times to make sure there is parking for it in their lot.
2. Continue West on I-90 and stop off at the Little Big Horn Monument, Montana (Custer's Last Stand). This is located about 25 miles South of Hardin, MT. They have limited parking for RV's, but don't go with an RV during peak hours.
3. Continue Northwest on I-90 to Livingston, Mt and either go North to Glacier National Park, or South to Yellowstone / Grand Teton National Parks (You need RV reservations anywhere in these areas).
From there, decide how you want to go back home. That's plenty enough for 12 days..... :-laf