There is a Cabelas in Billings--they usually have RV parking and a dump. There is a large area to stop overnight just North of the freeway in Missoula, Exit Reserve St (South of the freeway is a large RV company with services). A forest Service CG is just off the freeway at about MM 60 on I-90 (at the rest area) if you're going that far West.
O K, I'm cheap--but, if yer looking to just stop for the night, these will work.