When you're out in nature you don't give a damn about looking and smelling like a homeless person. Being able to bathe and groom whenever you want to be around civilization, where the gyms are, is the point.

A marine battery and portable solar panel are not mutually exclusive.

When you've found a sweet spot and have no intention of driving for at least a week, it's less than ideal to repeatedly idle an automobile engine just for charging your battery. Most of the energy is being lost to waste heat, it's a horribly inefficient method and annoying to boot. But sure, charge your batteries off the alternator while you're driving places as well. Everything should get topped up whenever you drive somewhere.

Most state parks I've stayed at around california permit 14-day continuous stays. Enforcement hasn't been especially strict in my experience either unless it's a busy time of year or there's an associated nuisance.

If you don't drive every day, then if you ran a laptop all day on your battery you are stuck the next day. Of course, the answer is don't run a laptop all day.. but that just changes the variables a bit because if you want to stop somewhere a bit longer is there enough power now? Also, running engines is not great for charging batteries which need to float a while to get full. A solar panel can make a huge difference..

Yes, agree, the answer is don't go live in a car and use the laptop all day every day. That wouldn't be very fun. :) Get outside if you run out of battery.

Having access to all the comforts all the time is for the part of your life spent in civilization.