What is roughly the lowest possible monthly cost of living?

I'm planning a potential very lean fire van life in case the job market collapses/war breaks out/etc. Ideally it would never come to pass, but in this day and age, prepping doesn't hurt.

Costs would include the van, food, clothes, internet, gas, insurance, van maintenance... is there anything I'm not thinking of? Any major recurring costs (other than clothes and computer stuff)? I don't need to pay taxes, right?