Another voice saying 400 to 500 miles a day is too much. Maybe not the first day or two, but it doesn't take long for that kind of schedule to beat you down to the point that it isn't fun any more.
I am 73 and my wife 70. We plan on 275 to 325 miles a day, knowing that there will be some much longer ones and some a lot shorter to end up where we want; when we want. I start from a "primary" destination and work backward to a start date then finish planning forward from there.
Another hint for long trips (We will be going from Orlando to the Bay of Fundy and back, 5200 miles, over 6 weeks in July and August.) is that we never drive more than three days in a row. After the third day of driving (or sometimes just two), I insert a layover day as "down time". If there is something interesting to go see or do, we do it. Even if we are in a place that doesn't particularly interest us we rest, clean up the inside of the trailer, do laundry or grocery shopping, or just sit under the awning and watch the people go by. If you make 300 miles for three days and rest one, you are still averaging almost 250.
By the way, over the last nine years we have spent about thirteen weeks at Yellowstone/Grand Teton. We will probably take the grandson out there next summer. Stay at West Yellowstone or Gardnier and just take the truck into the park. The CGs inside are not really big trailer friendly, although it can be done. It is just not worth it.
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