If you want to 'earn' your tank (i.e. via six stars) the best method is to run the police missions on all three islands so you have 6 stars at the hideout, and run the fire missions so you have the flamethrower.
Get police attention at your Staunton Island hideout - battle out with the cops and the FBI (helps if you have rocket lauchers and machine guns, but the flamethrower is a BIG help).
When the army arrive, flame them, and then run back down to the hideout and grab the six stars to reduce your wanted level to zero. You should be able to run back up and grab an abandoned tank.
It sounds a hell of a lot easier than it is, but it is acheivable!