I had a very similar project in January.
As Peter says sides are not usually a problem, treat as a flat panel, measure up from chassis after checking tank is parallel with chassis. If not parallel have to decide on reference points to give best result.
Some of the trucks had the joins right in the middle of the graphic and then they have to be slit in the right place.
Rear of dome is biggest problem if laying a large area of vinyl.
2 main problems are getting vinyl to lay without creasing and getting it straight.
Straight line. Again measure up from chassis at each side then chalk line accross, or find centre point with a plumbline and then use a rule with level.
I had to do this design, and asked all the people I know how they would tackle it.

I was lucky that the ends of the tank were painted green so I could just cut the other colours seperatley.
Suggestions for laying the green if it had had to be coated first were to either lay an oversized cast vinyl stretched to fit the curve and then trimmed to the correct shape, vehicle wrap the whole dome, or PAINT IT.
The customer was concerned that the oval would be misshapen as he had had 2 sign companies before try! (a bit of pressure then to get it right).
I applied the oval seperatley by first placing it on a complete piece of application paper which was trimmed tightly to the edge of the oval and after lining it up taped the out side edge to the tanker. I then trimmed out the application paper from inside the oval to leave a ring which would lay flat on the dome, after cutting at one point and removing a few inches then overlapping the join by a couple of millimetres.
Letters went on fine individually. I found centre point of dome then used my ruler with level to mark the base line, checked by eye before final application. However a longer word would be more noticably smiley and more adjustment needed.
Good luck