This link is a must read:
Yellowstone, Grand Teton, and the Beartooths
Make reservations now for Grizzly RV Park in West Yellowstone, and Coulter Bay in the Grand Tetons. We stayed at Red Rock RV Park in Island Park, Idaho. Less expensive, but 25 miles further away, which adds 50 miles every day to your 100-125 miles loops in the park.
If you are going to spend 3-4 weeks there next summer, you will get awfully tired of driving around in your toad. Might as well get as close to the action as you can.
I was disappointed in Yellowstone this past July. Way too many tourists and traffic jams, not enough wildlife. In my opinion, Glacier National Park is way more spectacular and far less trampled.