Hi Filip,
nice start on the Gotha.
The fuselage passageway was covered over on the G.V as the 2 fuel tanks filled the space between the pilot and rear gunners positions.The fuel tanks were positioned in tandem,but alas,there is no photographic or illustrative evidence of their shape,except for a rather indistinct GA drawing/plan that is available to view on the Russian 'Flying Machines' site that just faintly shows their postion in the fuselage.If you can get hold of them,Scale Models from Feb/Mar/April 1976 had an excellent coverage on the Gotha bombers by the late great Harry Woodman,with some good photos of the tunnel covered over.Some of these photo's are on line,just Google 'Gotha bomber',but ignore the Wikipedia fuselage layout!
Dave.