propane tanks

[Ryland]

Unless you are good at designing a drain back system you are going to need anti-freeze and a heat exchanger so at that point the quality of the fluid in the tanks doesn't matter.
Anyone who deals with boilers will sell you a boiler rust inhibitor that you add to the fluid in the closed loop of the system.

water tanks for drinking water are often glass lined or stainless steel, old ones were galvanized, LPG is dry so rust is not much of an issue in their tanks without a coating.