I can't say how crowded the park will be ; mid summer ; no matter what time you arrive, I was there in October and it was crowded, there is limited RV parking at MR, you can go to Google maps then satellite photos to see the area.
Parking at the Crazy Horse monument was not RV friendly, we had just taken our truck ( 5er was in Rapid City ) and had issues finding parking.
We only drove the area in our truck , but can't say I noticed any roads that could be an issue for a small RV & toad. If memory serves , I passed RVs on all the roads I traveled in the area.
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.