A modified pipe rack, using a few steel angles and some fiberglass or plastic panels, makes a great sleeper on the roof of your van.
I made one out of a minitruck cap using a plywood base.
Just bolted the thing right to the plywood and the cap locked up nice and tight to the bottom. Some sealer before it was bolted in place and the carpeting never got wet no matter how heavy the downpour.
You could liberally use some foam insulation inside the thing and if you had a few inches on the walls and roof, your body heat would be enough even in 20 degree weather. Colder than that, an extra battery and one of the simple electric heaters for a semi cab would allow you to snag 5 sleepy hours and not freeze to death.
It would look no worse than having ladders and a toolbox up there, which they are seen with all the time.
You enter from a ladder on the back doors. Unless it is pouring out, this setup really does work well.
Whatever you do, do not think a cheap old aluminum cap will be worth the money you save. They leak. They all leak. Fiberglass is the way to go!
We had to weigh it and with the mattress/blankets/sleeping bag/motorcycle battery/laptop/TV/pillows/water, the whole setup added 320 pounds to the van.
WalMart sells a long, shallow plastic bin with a cover and it was paramount in this systems success.
I think the total cost was under 500 and it can go from van to van.
We took four pulleys with steel clothesline, tied them to an oak tree in the yard and it can be dropped on the van precisely in a matter of minutes.
I have an extra rack for the roof I might be talked into selling.
I am shocked more people do not do this with space being so valuable in a cargo van.
(BTW, MPG in a 350 Ford with a 351/C6 auto suffered less than 1 MPG with a 68 MPH ceiling.)