I follow the van dwelling scene a bit (got a site in the space: https://vanspiration.com) - it seems that many beforehand overstate the need for a toilet. Those moving a fair bit amongst urban areas often make-do with public or business toilets and go months without needing to use their in-van solution. Those out in the bush are digging holes.

Many will typically have a pee bottle, and then a bucket double-lined with plastic bags and filled with dirt or cat litter. There are contoured lids that fit over common bucket sizes and make something of a seat.

There are loads of cassette-type portable toilets too.