Spring Creek CG is near Big Timber, MT and is a very nice place to stay. It's very clean and well maintained with some of the friendliest staff you'll find anywhere.
We stayed at the KOA in Bozeman (great museum there, btw) then drove a short distance to Red Cliff CG in the Gallatin NP. It's right on RT 191 and I'm pretty sure there are sites plenty large enough for your rigs.
W. Yellowstone has a lot to choose from. We stayed at Grizzly's for a night before entering Yellowstone NP.
It looks like you've planned a great trip!
