To reply to your question Pete
I think that the manufacturers use SS tanks because they have better margins (steel is VERY CHEAP). I'm in the shipping bizs and I know.
In Falcon they use heavy duty plastic tanks. They can fit in SS tanks but is not recomended.
For SS tanks should go between deck and hull, then the deck is sandwiched and you just get a small opening for inspections purposes. If anything goes wrong with the tank or the lines you will have to cut through the deck.
Now the boat is very light (380 kgs without engine) and is constructed with advanced technology materials. If you cut through the deck you will compromise the seaworhthiness of the boat.
So, Falcon in SAfrica recommends Palstic.........and as the moto says
PLASTIC IS FANTASTIC!!