How much does it cost to live in a campervan?

I have a plan to travel across the states in a Sprinter van that I will convert into a campervan. I have many connections with people who are willing to work on the van (some do woodwork, some electric, etc) in exchange for specific items in my household for when I sell it. So basically, I'll have a camper that is completely self contained (solar panels, induction cooking, chargeable battery/generator, composting toilet, pump shower, etc) in exchange for furniture that I saw as junk and would have had to pay to discard. Talk about saving money. However, I also want to be self contained and not have to rely on anyone to help pay expenses. So I want to have money ready for a year worth of living before I start my adventure.

How much should I save?

Excluding electricity and water, what would my expenses be and what do you estimate they would cost me per month?

I sing and play guitar and the piano, however I know street performing is not a reliable job; how can I make money on the road?

Thank you in advance :)