tanks is perfect shape.
In the past, steel were good for only aboutc5 years. I guess they would have done better if kept out of the occsional sea water splash.

The new transparent plastis ones are best, if they fit your locker.

No experience of diesel in cans becasue we carry about 700 litres, say 200 US gallons. I see LOTS of plastic cans used by other boats. Buy good qulaity, intended for diesel.