Gosh Montana is such a beautiful state, hard to name just a couple. I have yet to go to Glacier NP, we were going to go this year but had to change plans but that certainly is a must. I will second Custer Battlefield if you have any interest in history, specifically military and history of the west. Caution though it is quite a bit of a drive off the beaten path, so you keep that in mind when planning. I'll also second Bear tooth, particularly Bear tooth pass, a beautiful drive & Cody is worth a stop. Sorry I cannot recommend any camping sites as it has been so long since I traveled through Montana I cannot recall names and all the camping I did was tent and in off the beaten path.
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