I'm with Peetza on this one. Living in the hills and having a junkpile has let me shoot many many metal "things" if you are talking a bulk propane tank of 250 gal or so I have never seen a shotgun round whether .410 or 10 ga that would even remotely penetrate it. Even the smaller 20# bottles with a shotgun at more than contact range would notconcern me. The tank should have a metal cover over the controls/tubing etc. which are much more likely to be damaged than the tank itself. Honestly I would think that you have a better chance or having a misfire or a mechanical breakage of the firearm (shotgun) than you have of busting that tank with a shotgun.
Old waterheater tanks, and pressure tanks from wells can take an amazing amount of shooting with little damage to show for it. However if you are curioust some old black tipped 30-06 ammo will shoot a hole through it with an ancient springfield 1903!!