I guess the roof only needs to be big enough and long enough to accomodate a bed; modifying things to that there was one would be doable. And yes, it's been ages. I got sidetracked; bought a big old 22 foot Merc and converted that, but last year I got sick of it; too big, got turned away from campsites some places for "not being a proper camper", a bit unwieldy, and I couldn't justify keeping it on the road because I only drove it 3-4 times each summer. Now got a Daihatsu Hijet which is fecking brilliant - comfortably sleeps two in the back (no space for anything else though, all the camping gear lives under the bed, I put up a Coleman classic awning, and everything but sleeping happens outside!), and can carry six people, or, I've discovered, a piano.
Anyway, I could now afford a £1000 Commer, but I don't have time or money to spend on it, but a career change has been forced upon me by way of redundancy, and one of the irons that I've got in the fire would get me enough income to comfortably buy a decent one, get it the way I want it, and keep it in good nick. We'll see!