Complete beginner first time questions for a USA road trip

Planning on a West Coast and south Midwest area. I have no idea what I’m doing. Would appreciate some answers.

Based on the rough circle I made on the map, how many weeks would you plan for the trip?

I’ve never slept in the car before. If I lower the backseat I guess I can kind of lie down on a thin mattress? Or should I sleep in the front seat? It’s a regular sedan.

I heard that people sleep at Walmart or hospital parking lot. Is this actually a good idea?

I do get paranoid about safety sometimes especially at night. Do I need to bring some sort of a weapon with me?

Do I need to do anything in advance to spend time and possibly sleep within national parks? If I’m visiting multiple, should I get the annual pass? Is this something you buy online or in person in any national parks?

Do you cook at all or just buy all the meals?

Is it worth it to get planet fitness? Just so I can shower any time I want almost anywhere?

If you do plan on staying at the hotel, do you reserve or is it fine just walking in?

Is it better to comprehensively plan out where to go and where to sleep before departure?

Sorry for the long list. Thanks in advance.