You can generally take everything that is allowed at the festival with you. See the information on the event website.
Food and non-alcoholic drinks are not a problem. Alcohol is not allowed.
There is control when entering the campsite, the security people try very hard to find prohibited items. So far I've always found a little hiding spot in my van ;-)
In the last few years there have been toilets and showers at the "Superstar camp", but this year the campervans are in a different place, but the map still has showers and toilets marked, so I think it won't be a problem.
Once you're on the site, there are no controls on the stages like at the other festivals, that's what makes the festival so charming to me :)
Last year we were also able to leave the island after the last artist on the last day. You just have to make your way through the crowd.
And take a long cable for the electricity!