Adult ONLY RV parks. ?

I am not familiar with any 55 plus parks in Wyoming or Montana. What you can do is look for parks with minimum facilities for children, extra fees for more than 2 guests per site and other things that might not be attractive for families. A lot of parks in these states (mine included) are what can best be described as family tolerant, rather than family friendly. No playgrounds, no pools, no game rooms or arcades, no campfires, not bargain priced, and will tell you that there isn't a lot for children to do in the park if you ask. Personally, we are fine with the KOAs in the area taking in the families and leaving the traveling adults to us.