I am the ex-gas engineer wot said I don't think they rust from the inside, because LPG contains a small amount of oil. I think the following, but I have never cut a tank open:
Oxygen causes steel to rust.
When it is manufactured obviously there will be air, with oxygen inside it.
Then it is filled to 80% full volume-wise with liquid petroleum gas.
The petroleum is refined from underground oil.
The liquid gas contains a small percentage of oil, because LPG is liquid petroleum gas.
This is different to LNG which is Liquid Natural Gas, i.e natural gas that has been compressed to a liquid state. The same stuff that your central heating boiler uses, and that doesn't contain any oil.
There will never be any oxygen inside a tank after it has been filled once and emptied. As it is emptied, the space is not taken up with air, it will always have the pressurised gas above the liquid gas.
Therefore there will never again be any oxygenated air inside the tank.
Some small droplets, or particles of oil will remain inside the tank, and my theory is that combined with the lack of oxygen, the tank won't rust inside.
I know some of the above to be true, some may not be and I stand to be corrected by anyone who has cut a healthy tank open.
Also when I last had a hab check done I asked the chap about the life of a tank and he said the legal requirement is to have a fixed tank inspected and certified every ten years. He said this is too expensive on a motorhome tank so it is cheaper to remove it and fit a new one. If the tank is found to be older than ten years it should not pass the habitation check.
As for straps going rusty inside the plastic coating, I recently got a bigger fresh water tank from CAK tanks. They supplied a set of stainless steel straps inside the plastic coating, and I can't see that deteriorating.
