If you're refurring to I-90, that will bring you to the north entrance to Yellowstone where we stayed a few years ago outside a town called Gardinier. There is 3 or 4 RV parks in that area all within 15 miles of the north entrance. Check your Woodalls or Trailer Like Directory. Next time there, I plan to enter from the east side in the Jackson Hole area as I've been told it's much closer the heart of everything going on in the park. Enjoy your trip!
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You would need to explain what "best" means to you when it comes to the preferred route. Route 20/26 is a LOT more scenic for coming in from the East and coming over Bear Tooth Pass on 212 is more scenic then 20/26 but traveling the Interstate to route 89 or 191 would be a lot faster and easier. Much of it will depend on just where in Yellowstone you are going.
Can't help with RV parks because I don't use them but others will have recommendations for you I'm sure.
Good luck / Skip
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Well Hwy 212 to Crooke City goes across the Beartooth pass over 10,000 feet and lots of switchbacks. That would leave you at about 80 miles from the Geyser Basin.
Going thru Cody, I didn't notice any CG once you leave Cody. Cody is over 100 miles from Geyser Basin. And bear in mind once you hit the park that the speed limits are low and the speed you'll actually get to travel at are even lower much of the time.
IMO staying anywhere except in the park will doom you to spending a lot of hours coming and going every day. Even with a in park CG you'll be doing a lot of driving, this is a -big- park.
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Stacy - I believe that your best approach to Yellowstone is taking US 89 south from Livingston; you will enter the park just south of Gardiner and then the first place to visit is the Mammoth area. If you need hookups, probably the best area is either in the Gardiner locale or go over to West Yellowstone where you will find several RV parks that are nice. Unless you are well equipped and comfortable with mountain driving, I don't think you should attempt to enter the park via US 212 over the Beartooths; that's a steep winding road.
Mammoth campground IN the park doesn't accept reservations. So if you would stay at a RV park just outside of the north entrance the previous night and move into Mammoth the next morning, you would get a site. It's a very nice campground. Staying IN this huge park is the best way to go. Once inside the park there's a LOT of driving involved. Have a good trip!
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Easiest entrance to Yellowstone from the east is I-90 to Livingston, Montana and down to Gardiner/north entrance. No climbs of any significance on the interstate and US 89 from Livingston to the North Entrance feels almost flat. If you want full hookups and want to stay near the Park, only the North Entrance and the West entrance really work. The south, east and Northeast gates do not have full hookup parks anywhere near the Grand Loop, with is the road you will mostly travel within Yellowstone. For the North entrance, there are two RV parks in Gardiner, Yellowstone RV Park and Rocky Mountain RV Park. We love Rocky Mountain, brand new restrooms, fantastic views, 50 amp full hookups including Wifi and cable. Pricy and the sites are adequate, but no one would consider them huge. Yellowstone RV park is a little out of town, does not have 50 amp and is mostly filled with Yellowstone Workers, but not bad. Yellowstone Edge RV Park is very nice, on the banks of the Yellowstone River, but is 35 miles one way to the north entrance and is not close to anything. They also do not have cable TV. I would highly recommend them for a fishing location, or a place to spend a month or more, but as a base camp for Yellowstone, 70 additional miles a day gets to be pricy and inconvenient. Gardiner has shops and restaurants, but is less commercial and developed than West Yellowstone. West Yellowstone is the place to be with kids if they need additional entertainment, McDonalds etc. In West Yellowstone, the usual suspects Grizzly and the KOA reign supreme. In the park, the only hookup option is Fishing Bridge. No Cable and No wifi, however. It is also further away from Old Faithful than West Yellowstone, and Further from the Lamar Valley (best animal area) than Gardiner. No place is close to everything, so plan on lots of windshield time.