When were you planning to come out, southbound? If you're looking toward next year, then you're in the right timeframe to be making plans. If this year, you had better hustle, since most of the CGs in Yellowstone close soon, some of them within a few days.
Colter Bay RV Park is lovely, very wooded, but it does take big rigs. Another good CG for Yellowstone is Grizzly RV in West Yellowstone. Not much in the way of trees, but a neat, clean park, and only a few blocks from the west gate of the park. Red Rock
is nice & has more of a country feel, but it is a bit of a drive to the park entrance. Tighter sites than Grizzly too.
I agree to include the Tetons with your Yellowstone visit. One entrance fee includes both parks for a week; they are literally right next door to each other. Depending on how much time you're wanting to spend on the entire trip, I also second the recommendation for
the Black Hills, and if so, spend a night at Devils Tower. The KOA right at its base has a spectacular view! If you're interested in the westward trek of the pioneers, make sure to go one direction across Nebraska; lots of interesting pioneer sites along I-80. If you can do w/o full hookups for a night or two, consider some of the state parks along the interstate there - some decent electric-only sites among them, and during the week it shouldn't be too terribly crowded. If you do this route westbound, consider taking off the interstate to US 26 in Ogallala, which will take you along the Oregon/Mormon trail route by some of the pioneer landmarks (Chimney Rock, Scotts Bluff, etc.) Guernsey SP in eastern Wyoming has campsites and also has some of the pioneer wagon ruts permanently carved into stone. Should be worth a stop.
If you have a MH (which it sounds like), you will want to bring a toad, rather than drive the MH around all the time. Lots easier! Certain roads (I'm thinking Needles Hwy in the Black Hills would be one of them) are not to be done in an RV of any kind.
