The heart-warming tail of an Ecomodderer who put a roof-top tent on his TDI New Beetle and was dismayed by the MPG hit, accepted some friendly persuasion, and achieved better than OEM mileage with a hitch-mounted rooftop tent.
ecomodder.com/forum/showthread.php/compact-camper-alternative-new-beetle-tdi-hard-mounted-26915
[explanatory pics victim of a fickle image hosting site]
You might achieve a similar result with a long, skinny hitch-mounted fiberglass pod.