It's wildly variable what resources someone in that kind of situation might have.
There was a guy living in the storage unit next to the one I have in Austin (for my stuff from when I moved out of a house and back into an RV). The folks who ran the storage facility let him run an extension cord to the nearest power; several people actually were living in that facility, despite it being illegal. So, in his case he did have electricity, and I believe he had a mini-fridge. We talked several times and he seemed to have a pretty reasonable setup. He even had a space heater for when it got cold. I never saw the inside of his house (he'd built a wall and second regular door behind the rolling garage style door), but he seemed to be reasonably comfortable and had regular work really close by (since he only had a bike, proximity was important). The last time I was there, he'd moved out, and the person working didn't know where he'd gone; I'm hoping he was doing well and got an apartment rather than ran into harder times.
Similarly, I've seen people use one of these to get electricity inside a storage unit when they didn't have particularly understanding landlords: http://i.walmartimages.com/i/p/00/04/31/80/63/0004318063152_...
But, people in cars or vans probably don't have power. Some RVers have solar panels, and also RV fridges often also run on propane (though they aren't terribly efficient in the old RVs...I go through about $15/week running mine on propane). Propane fridges are very expensive and very fidgety and prone to breakage over time, however, so it's not uncommon to see the propane/electric dual-mode fridge replaced with an electric-only model in the cheapest old RVs. So, that limits the use of the fridge to times when it's plugged in.
I know someone who lives in a bus that she converted to an RV herself, but she didn't put in a fridge (yet), so while she has a stove, if she's not parked near a grocery, she is very limited in what she can cook. Some things keep well without refrigeration: onions, potatoes, winter squash, etc. She has potted plants for tomatoes and herbs, but she's much more intentional about her situation. She moved out of an apartment and into the bus by choice, not by necessity, and mostly enjoys the off-the-grid lifestyle. She made one of these for when she's in the desert: https://en.wikipedia.org/wiki/Pot-in-pot_refrigerator but those only work in very dry climates and aren't cold enough for many foods. Obviously wouldn't work in most places...and if you're in a city, you have to be self-contained and very discreet about where you're living. Can't really spread out and put out pots and stuff when parked on a street.
