There's one or two on here (links at the bottom.)
I seriously considered one, but I found more drawbacks compared to a bus.
Step vans were equally expensive compared to a bus, and the drivetrains I found weren't guaranteed to be any better. Bus or step van, both are designed for local driving and stop and go - not highway cruising. Lots of wheezy engines, and non-turbo diesels. Plus, on a van, you've got the engine half in and half out of vehicle, so service can be more challenging. The step vans were more expensive, and higher mileage.
I wasn't happy with the seating arrangement in the step vans. The drivers area is usually walled off from the back, creating a cramped front end. If you put a reasonably comfortable passenger seat in there, then there's no stairs/easy way in or out of the van. There's still a wall between the front and the back of the truck - so if you have passengers/friends in the back, you can't hold an easy conversation with them. It also means that you can't count on the drives seat or passenger seat to be part of the living space when parked. You could put a better seat in here - but there's no room to recline it.
While a step van is still mostly all steel (or aluminum), they're not as sturdy as a school bus. They might not pancake in a rollover, but they're still not as good as a bus.
I also looked at the dimensions, and a lot of the step vans aren't that big. They've got some extra height, but not a lot of extra width or length. If you're by yourself, or a couple, I'd imagine it would be fine, but otherwise it might be a bit cramped. Also think about where you'd put the bed or the bathroom - if it goes sideways across the van, then you need the front stairs to enter/exit.
Imagine the bed on the left, that's your kitchen on the right and. . . that's about it.
There were a few nice things - like they'd be super easy to insulate, because the walls and floor are mostly straight. You could easily cut windows wherever you wanted. A power lift gate would make a sweet back deck. If stealth is your goal, you certainly can't beat a step van.
Also, while I know that step van is only an example. . .it's $16,500. You could buy a LOT of bus for that money. (Both in quality, equipment, and literal size.)
Here's one on here, but they haven't updated since 2013.
It's an awesome polished aluminum finish too!