look in the bottom right corner you'll see a screw on lid in a oval shaped peice of rubber with bolts all the way around it. Its on the center line of the vehicle. behind the engines.
Bad news
Trumpeter didnt mold that on at all. I've checked my model and cant find it anywhere
If you look at the model you will find that there is a fuel cap moulded in it ,its the oval shape on the engine deck,Darktrooper has got it right for the fuel tank if you lookat the photo the engine deck is opened up.