I've added a rear compartment to my 24' twin.
I don't know how common this is, I couldn't find any other references.
I'm converting to a rear bed so I wanted to use the space under the bed efficiently.
I've actually put a generator locker in the side as well.
A little tricky as the side must be curved.
The reason I'm sharing is that I couldn't find any commercial hatch doors I liked so I made them out of the rather nice aluminum extrusions the interior cabinetry is made from.
There are a few more details on this
page of my blog
Travels with Geordie