My van has a stock Ford roof, and a Yakima rack. I plan to put my Thule Evolution box on the Yakima rack, on the passenger side. An Evolution box is hinged on the side, not at the rear, so access to all the contents is easy...if you can reach the box!
A ladder on the rear door or the rear side won't do much good, unless the roof is strong enough to walk on (is it?). And even then it would be awkward.
I could carry a three-step stepladder, which would solve the problem of reaching the roof box, but then I'd have to store the stepladder.
I could put a ladder on the rear door, and then use that as a rack to carry the stepladder that I use to reach the roof box, but that seems inelegant -- in fact, downright silly.
Any other ideas? I've seen roof boxes in a few of the galley photos -- how do you reach them?