Idaho is a very long state, from north to south. Do you know what part of the state you will be in?
In southwestern Idaho, for a really neat place that is well off the beaten track, try the City of Rocks National Reserve and its adjacent Castle Rocks state park. Castle Rocks has reasonable RV camping-- there is a lot of dispersed camping in the City of Rocks but some of it is a bit sketchy for an RV. Nice hiking trails for the non-rock climbing contingent.
Plan Your Visit - City Of Rocks National Reserve (U.S. National Park Service)
In northern Idaho, try Priest Lake state park, as well as camping in and around the resort towns of Sandpoint and Coeur d'Alene. If you want a waterfront RV resort type of experience, try Blackwell Island RV park just west of Coeur d'Alene.
In Wyoming, for sure: Yellowstone and the Tetons!
Grand Teton National Park (U.S. National Park Service)
Yellowstone National Park (U.S. National Park Service)