Montana is no problem as there are no large cities and the interstates are lightly used.
I have used freeways where it wasn't really legal in order to avoid a long detour. A good example of this is in New Jersey where US9 uses the Garden State Parkway for 2 1/2 miles over the Mullica River near New Gretna. Cops passed me both times I've done this but didn't stop.
I've been booted off freeways a couple of times but with my dumb, innocent, polite demeanor the cops just checked me out and sent me on my way.
From El Centro CA to San Antonio TX I traveled on I8 and I10 about 80% of the time. Generally not very scenic but usually the only practical choice.