Is it correct that we can leave our car parked here overnight while we are camping overnight at a campsite in Voyageur national park? We would leave it there for four nights. Our campsite is a 2.5 mile paddle from the campground

Park at the lot by the dumpster or boat landing, in the campground you would have to pay for a site. I would check with the voyagers visitor center to be sure.

Yes there is overnight parking for free right near the campground entrance. You can absolutely Park your vehicle there. There is nowhere to park inside the campground for free. As the previous Hamilton stated you would be parking right near the boat launch anyway.