You’re not going to suffocate in a parked car just because the windows are closed if that's your concern. Cars aren’t airtight, so there’s still some air exchange even with everything shut.
The real risks are heat and carbon monoxide. Don’t sleep with the engine running in a garage, under a shelter, or anywhere exhaust could build up. Also make sure the tailpipe isn’t blocked by snow, mud, or debris.
Cracking a window can help with condensation and make the car feel less stuffy, but you don’t have to leave one open just to breathe. You can buy cheap stick-on rain deflectors on Amazon that go above your windows and work pretty well.
Also, if it’s hot out, a closed car can heat up fast, so be careful there.
For a road trip, I’d just keep it simple. Tent when you can, car when you need to, and use places like campgrounds, legal rest areas, Planet Fitness, and libraries as you go. Just check local overnight parking rules because they vary.