Really Silly Question..

[QP4Me]

...but where is the petrol tank on my 2005 QP!

 

[RSM Masser]

No such thing as a silly question - if you don't know the answer.
Sadly neither do I (know where the petrol tank is) at a guess its near the back of the car (the filler cap should give a clue) most likely in the centre for even weight distribution, behind the back seat and in front of the rear valance, holds 20+ gallons so it wont be small.

 

[TridentTested]

I don't know, but I would guess it is between the boot and seat. No space underneath due to the transaxle gearbox, the boot is short in depth and there is no ski hatch are the other clues.

 

[drewf]

I agree - looks to me from the exploded parts view that it is behind a panel at the rear of the boot, and ahead/under the rear seat. It appears to be fairly central to the filler neck.