I've used an oxygen scavenger http://tinyurl.com/kh5c6lb
for guns, tools, and casr-iron pots/pans. Just a packet or two and a well-sealed plastic bag - no rust. EVER! I use the same product for storing long-term food.
I imagine it would work the same on the interior of a gas tank. Clean and dry thoroughly, throw some oxygen scavenger packets into the thing and securely tape up the ports.
RUST IS OXIDATION. WITHOUT OXYGEN THERE IS NO RUST.
[This message has been edited by notaguru (edited 03-06-2014).]