I knew people who live in vans in San Francisco, which is fairly mild weather most of the year.
They shower at work, in gyms, I know one person who would "visit" hospitals and shower there...borderline ethical but not a huge deal since hospitals are already heating that water. Years ago SF had free showers near the wharf for people who swam in the Bay...not sure they still have that though.
One guy I knew did it because apartments in SF average about $1800 a month. So he figured he could bank that amount.
The only issue was having to go stealth mode for sleeping. During the daytime, there are a thousand places you are free to hang out, cook, etc. But one candle at night will alert a neighborhood that you're 'camping' and it can get ugly, fast.
While I know MMM advises against fast food, I know one person who would get a cup of yogurt with granola for breakfast, and a side salad at Wendy's for lunch, and something simple from the store for dinner who did manage to make it work: he worked full time as a house painter. He saved over 3k each month, by his estimate, and this was 20+ years ago.
It's not as easy as you might think though. Even people who know/love you will look askance at you. Dating is a challenge. It's illegal to sleep in a vehicle in most towns, and cops can be cruel. It can be dangerous: like the homeless, you're considered an easy mark as you probably won't call the cops...
The best thing to do would be to figure out if you can psychologically deal with the social stigma, then go undercover investigate people who are already doing it in the area you're thinking of living/parking. These people can be secretive (for good reason) but if you do manage to befriend someone just ask them where they're going for showers, etc.
Good luck on your decision. Please report back.