RV travel is usually slower than by car - you'll find yourself going about 60-65 on the Interstates, at most. I'd suggest:
Leave Mpls a 9am, get to Badlands NP between 5-6 pm. (Park campground, or a pretty nice KOA very nearby)
Badlands to Custer - (after a morning in the Badlands, lunch at Wall Drug) - another 3-4+ hrs, depending on traffic.
Leave Custer 8am, arrive Jackson area 6-7pm.
Split your Yellowstone portion into two areas - south side (Jackson or Park CG if possible) then up to Grizzly in W. Yellowstone.
+2 on pulling a car, you do not want to be dealing with where to put a Class A while sightseeing in the park.
As noted, the northern route back through N. Dakota can be a change of pace, If you can budget 3 full days, stopping at the Little Bighorn is well worth the extra drive. Or Roosevelt Park in ND is a variation on the SD badlands, and worth an hour or two drive through. Note that in WY, SD and ND in non-tourist areas, campgrounds are a bit sparse.
While it is not to be missed, skip exiting the Park through the North via the Bearskin Highway - not a road to take a class A with a toad on! And if you leave via the East, take Rt 16 through the Bighorns. Both it and 14 are pretty slow and scenic, but 16 is a bit easier and faster with an RV. 14 could cause a fair amount of white knuckles! (And banish all thoughts of RT 14A!!!)
