One poster suggests using carbon dioxide or nitrogen. Great idea if you can seal the tank and pressures when heated (sunlight, summer weather changes) don't create a problem.
BUT---carbon dioxide is NOT inert. Any moisture will create carbonic acid and eat your steel tank.
Just be careful and save that tank.
For long-term storage I'd be inclined to shut fuel valve off tight and ensure no leaks and fill tank with highly stabilized diesel and try to seal cap from breathing.
Please post back how you proceed and results so we may all learn.