I really wish I was further along in my build so I could show you the cabinets that I am working on. I have a high roof, but I want small cabinets, so mine will be less like what you see in other's builds and certainly unlike what are available commercially. I hope you are planning to make them yourself. Given the special nature of them I think that is likely to be your only option.
I am making my cabinets from 8020 aluminum extrusion as many others have done. (references abound)
Where I make a big change is that my design criteria is much different.
1. more storage like everyone
2. small cabinets
3. don't carry much weight up high
I really thought about what I wanted after having an ERA Sprinter with huge cabinets along both sides. It felt like I was walking in a tunnel.
If you think about traditional motor-homes, cabinets can be up to 18" deep. Some people put an entire set of china in an overhead cabinet. Or fill the cabinet over the sink with canned goods. That is not my plan.
So, I am using 20 series extrusion which is very light and only 20mm square. I could just have easily used 1" wall square aluminum tubing. My cabinet might have socks, shirts, a pair of pants or shorts, etc. I'll probably go the Graphite Dave route and just use bins and a partial front rail instead of doors. I already have a bunch of bins from before because I couldn't see in the deep cabinets anyway. And the big cabinets were too tall most of the time. I haven't finalized my size yet, but you can imagine a handful of forest service maps standing in a bin set in the cabinet and that is how tall and deep it will likely be.
I took a look at the stripped interior of a low roof van and it looks like you can build a small cabinet that attaches to the ceiling rib and to the horizontal "rail" that run along below the roof ribs above the windows and have the cabinets hang down into the window space as much as you like. I would think that you could easily get up to 8"H x 8" deep without really causing any restriction in movement or even visibility.
You could also build very simply and lightweight with 1x2 and luan. (Before the trolls come after me -
I did this with full size cabinets in a motorcycle race trailer that had to be very light. Simple 1x2 frame and luan sheathing. Self tapping screws through the 1x2 into the trailer frame. Cabinets were packed solid with gear [not tools] and 20 years later and many thousands of miles they are still solid as a rock. )
I say let your imagination be your guide. Cut up some cardboard boxes and mock up your cabinets. You have to figure out how to mount them, so mount the cardboard and see how it fits. Hitting your head on cardboard doesn't hurt so bad. LOL