I could be wrong, but don't the bunks in a Dormobile take up just about the full length of the roof and you get into them by climbing between them?
That means (if I'm correct) you couldn't replace them with a full width bed as you wouldn't be able to climb up
The Carawagon roof is a bit longer and you climb into the bunks (if both are out) from the back end as there's still space between the end of the bunks and the end of the roof. Very useful for standing to cook, wash etc. while the bunks are out

That means you could cobble something like that together in a Carawagon
You might be best making something like an
X-Panda. This one shows it on a SWB but you could use the same idea. You could make it long enough for a mattress and some space to climb up rather than climbing up outside.
There are lots of other weird and wonderful ideas on Mr Leadfoot's photobucket page
HERE
At a show a couple of years ago, a couple visiting from Australia told us about their modification - they hinged the whole roof on one side so it lifted with a pram style hoops with canvas, and went right over 180 degrees so it stuck out on one side, supported by legs. That made a full size bed on the inside of the roof!
I did start planning to make something like that, and even got a vehicle to do it with (Steptoe, now owned and converted to a camper by Neil on here) but then a Carawagon came along
Hope that helps,
Robin.
Cheers,
Robin.