ledge construction

On another forum, a friend and I had discussed mounting a [url ="http://www.capricamper.com/"]capri model 42[/url] on a 4X8 trailer.
You'll note that because the capri is designed to fit in a truck bed, it has an overhang (I estimate about 16"). This overhang allows for a full size crosswise bunk while minimizing the footprint.
When I begain researching Vardos and Gypsy wagons, I noticed that they also had a "ledge" design.
Doing this on a trailer would allow you to have a crosswise bunk, a space for a galley, and seating, all the while putting it on a small chassis.
This strikes me as such a good idea, that there MUST be a reason why everybody isn't doing it.
So I'm asking, is there anything about the "ledge" design that is wrong?
You'll note that because the capri is designed to fit in a truck bed, it has an overhang (I estimate about 16"). This overhang allows for a full size crosswise bunk while minimizing the footprint.
When I begain researching Vardos and Gypsy wagons, I noticed that they also had a "ledge" design.
Doing this on a trailer would allow you to have a crosswise bunk, a space for a galley, and seating, all the while putting it on a small chassis.
This strikes me as such a good idea, that there MUST be a reason why everybody isn't doing it.
So I'm asking, is there anything about the "ledge" design that is wrong?