there are a lot of options, tbh. But really, as honest as can be, you could get any cheapo fifth-hand van from an online marketplace (eBay motors, Facebook marketplace, craigslist, etc.) and reno it into what you're looking for.
Roof extensions exist, you can add a few feet to nearly any cheap van on the market. But - at least in my opinion - vans are just too small to have a decent bathroom/toilet so you might have to cut your losses if you're going with a van and settle with a toilet that isn't perfectly what you're interested in and just get used to making due when you have to go number two. But from personal experience, there's little worse than spending your entire day smelling the fat log you pushed out after your morning coffee while you're too far from a dumping station for your brown water tank (or compost tank, or whatever). IMO onboard toilets NEED their own lil room with a door that shuts so you can isolate the potentially unhealthy smells from the rest of the vehicle - but that's just my opinion on that matter, live ya life.