I haven't been to Yellowstone recently but if I recall correctly, I would prefer to not drive a motorhome on the roads up on the north east corner of the Park. Also as others have mentioned, you will find parking a rig that size very problematic particularly in August.
How many days will you be spending in Yellowstone? If just for 2-3 days and if all you want to do is drive through and stop at only the major sites, you will probably be fine. If you are talking about a week long trip where you actually get to see more than just the areas that are within 100 yards of the pavement, then I would seriously consider getting a rental car and travel in three vehicles versus the motorhome.