There's plenty of vans that don't have any rear windows
My campervan (Promaster 2500) has rear windows, but it isn't like I can see anything out of them when I'm driving. Usually just leave them covered anyway.
I've only ever heard of a requirement for 2 mirrors. Those can be on the outside.
Varies by state in the us. Some require two mirrors and one must be outside on the left. Some are similar but on the right. Some states you need two outside if the one inside is blocked.
because you can see what's behind you? Via the camera, but that's just a detail.
It's possible to get _better_ visibility from a camera than a mirror. e.g. good placement, adapting to low light.
I've had plenty of car cameras glitch out or stop working.
To say nothing of getting crusted with road salt anytime you drive in winter.
I said that it's possible to get better. Not inevitable.