Stay Clean!Even if you did choose this way of living, most people, if they found, would consider you homeless. Unfortunately, homelessness is generally looked down upon in America. If you're not using a gym to have a place to shower, definitely stay clean and "normal" looking.
Truck stop showers, sink showers however you plan on maintaining cleanliness will help you avoid that stigma of being homeless. Further, living in your car could be an adventure, an opportunity to rise above or get out of your the ennui of "normal" life.
If you're not keeping up on regular maintenance, it makes you less stealthy. Your safety and belongings are packed up in that car, don't draw attention to yourself.
When I don't want to go to the gym and workout, I can usually draw up my hair and a deodorant stick can get me through the day easily. Even with a home people still have grungy days. On Sundays, when I have to be at work before the gym opens, a good sink shower in the library keeps me fresh.
Deodorant, a package of baby wipes, and shorter hair is all you need to stay clean and stay stealthy.