I sure sympathize with you, having driven vans nigh onto forever. I find cargo doors with windows on the passenger side mandatory for fully safe operation, and the lack of that has caused me many near-crashes which only luck saved me from. Since backing is your main requirement here, I'd suggest a waterproof cam on the outside mirror as best, though running the cabling and the reversed image you'll get from most dashcams will be an issue. If you look into how the Tesla car's cams show video, you'll get a good idea of what to expect, as those are mounted front looking back too. My current 'bus' is all-glass, but my work shelves cover the rear sides, and my 2 side-cams need to be augmented with 2 more rearward to have the almost 360 degree coverage I want. Those are just for video evidence, I don't use them for driving.
I would also highly recommend a large convex mirror over there too, with it adjusted such that the inner edge barely shows the side of your van which will maximize it's view to the side. I can't live without that, so much so that I have a spare one stashed in case anything happens to the off-side mirrors as you just can't drive a van safely without it, and factory mirrors are never wide enough in their view.
Phil