Commodore, LPG/Gas question?

I'm interested in a Holden Commodore VT for sale, and it has duel fuel. I rang up the other day to ask when the gas system had last been serviced and its due for a service now (been 10 years). How much does it cost to service the gas system, and also my dad said something about having to replace the gas tank, what would I need to get done and how much am I looking at?