West Yellowstone is going to be your best bet. IDK if your going to find any rv parks open under 4’ of snow in 0 deg temps, but there’s no lack of space in the winter to just park. People rolling in with RVs and big trialers daily.
Make sure you can reserve a tour to get into the park.
If you’re talking about real riding outside the park, the options are pretty much endless to the north south and west of the Park.