Fort Peck is a great stopping point if you're enroute east or west but it's not a destination.
The 'good stuff' is mid-Montana and to the West. It just depends what your interests are. We love trout fishing, hiking and boondocking so can't help with RV parks.
Places we've enjoyed are Livingston, Bozeman, Big Sky area, Ennis , Wisdom, Anaconda/Philipsburg, Missoula, Helena, Basin, Glacier NP, Columbia Falls/Kalispell, Bigfork, Polson, Libby
Google 'Montana, Things To Do' and lots of ideas come up.