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)
http://parksandrecreation.idaho.gov/parks/priest-lake
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)