since it has natural seams where the end caps meet the main cylinder ---
main body -- rounds sized to match the size of the soccer ball pan or the 1/2 sports ball pan (soccer ball one closer in curvature to the tank's) assemble standing up with good stiff buttercream holding all the layers together -- at least 6 to 8 layers. run at least on dowel all the way through.
then tip over on side onto a big ol' sheet of rolled out fondant. Roll it up in the fondant and seal the seam. Trim of excess at both ends leaving just a bit to make the seam with half domes.
make two soccer ball pan or 1/2 sports ball cakes --- while flat on counter, cover in fondant. and trim off excess.
using good stiff BC and dowels -- attach on 1/2 dome cake to each end.....
work the edges of fondant on dome and cylinder so it looks like an "on purpose" welding seam.
then add that little dome that covers the valve -- just a hollow piece of shaped gumpaste or hardened fondant.
for supports on bottom, I'd use carved styrofoam. and for extra support, I get a piece of plastic or stiff cardboard (covered of course) that could span the two supports and help hold up the cake. Could even just use a long piece of styrofoam, carve out a channel to hold the cake and then paint most out black leaving just two white sections for the feet.