If it's just to catch some rest while traveling that what they are for. If you turned it into a camping spot then the Highway Patrol might say something.
Most rest areas don't let people spend the night but if you're just passing through and it's late I don't think it's going to bother anyone as truckers do it all of the time. You could check with the Montana State police and ask them
Rest areas aren't for camping out but you can park and get some sleep there if you're too tired to keep driving.
RVs can park with the truckers. That's for anything with a trailer or a big RV. The rest area is there to get some sleep so that you are not driving while tired. Even cars can park with the cars and get some sleep. I've spent the night in cars, trucks and campers and never had State patrol bother me. They don't enjoy scraping bodies up when tired people crash.
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