Switch over to Polygon sub object mode and select the 2 end polygons as shown in the screenshot then using the non-uniform scale to scale them in either the top or front view so they are of the right size then extrude them out. (fig.14, fig.15)
fig.14
fig.15
To build the Gas tank drag out a box in the front view give it a Length Segs of 2, after that as before convert it to an editable mesh and adjust it as needed. (fig.16, fig.17)
fig.16
fig.17
For the Air Tanks the Oil Tank object will work just fine. (fig.18)
