As long as you’re confined within your campsite and not bothering your neighbors, you can sleep in a tent, in your car, in an igloo, etc. You do you.
Please be mindful of safety if you choose to sleep in a vehicle. Always make sure it is well ventilated and NEVER sleep in a running car (Carbon Monoxide is a very real danger!). And always be mindful of where your car exhaust is and make sure it is never venting into an enclosed area like a neighbor’s tent.