On a search for an opional luggage rack that can be tow ball mounted I found This
Its a handy rack that can be fitted to the tow ball & allow you to carry goods/luggage on the rear of your vehicle in the arodynamic void thus saving you climbing on the roof & reducing fuel consumption.
Limiting factors are the design weight of your tow bar but the bak rack can carry up to one ton & by using some additional straps to tour rear doors you can theoritically increase this as long as the load is safe & secure.
Also a great way on increasing space withing your camper/bus & not reducing on the amount of gear you have to carry around (like the kithcen sink the misus packed)