Since the popularity of Swap'n'Go gas bottles, long term durability of the coating has become less important as these are swapped regularly and any bottles that need re-coated are treated when returned for re-filling.  Swap'n'Go bottles comply with all Australian Standards for caravan gas bottles. 

See more on Elgas Gas Bottles for Caravans

In Australian Standard AS 2469 - 2005, the categories which are stamped (numbers 1 to 4) in a circle on the top of the cowall surrounding the valve assembly on gas bottles are as follows:

The numbers refer to the rust proofing treatment of the steel, not the thickness of the steel. Obviously, the thickness of the treatment will vary eg hot dip galvanising is thicker than powder coating.

1  Hot Dip Galvanizing

2 Abrasive blasted + inorganic zinc rich coating + organic zinc rich coating.

3  Sprayed Zinc Coating

 4 Red oxide or polyester powder coat.

The Australian Standard requires all gas bottles manufactured in carbon steel to have a wall thickness of 1.75 mm and be tested to a minimum pressure of 320 Mpa.

The only exception is gas bottles manufactured in stainless steel are to have a wall thickness of 1.5 mm, and be tested to a minimum pressure of 360 MPa.

Stainless steel would probably be the best for caravan usage, but they are likely to be quite expensive. So, when exchanging gas bottles, the number 1 in the circle will have the most durable surface. The number 2 bottles are also suitable for outdoor storage on caravans. The number 3 and the powder coated type with number 4 in the circle are better suited to be restricted to home use, however all grades are legal for use on caravans.