Hi There,
My old S8 has the LPG tank there. Due to a dodgy fuel guage and not understanding how the fuel pump worked, I had the petrol run out and the fuel pump burn out as it uses petrol for lubrication, so be careful to always keep some petrol in the tank! Audi wouldn't touch the LPG tank so I went to an LPG specialist and they wondered what the hoo har was. This also meant that I ended up finding the pump in the plastic housing rather than the really expensive stealer part, so despite them charging me £200 diagnostics and someone else doing the work, it was cheaper than Audi doing the job
Lee